This is a mini-extravaganza with a very unusual metapuzzle.

Context

This set of puzzles was written over Christmas break in 2018 as a MIT Mystery Hunt warm-up. The puzzles were designed to be straightforward, but the metapuzzle was designed to be harder than a traditional metapuzzle. In particular, this was created to put out feelers for doing something different with metapuzzles than normal.

Hints

Al Kaline

It looks like a geometry puzzle because it is. You can get through a lot of it by just using the Pythagorean Theorem. Start with solving for α.
The sine ratio can help you here.
Similar triangles can help you here.
The values of the greek letters sure are integers less than 26.

Andy Pettitte

These clues can be tricky to break into. The bottommost clue is one of the easier ones to break into. The phrase you are coming up with is not a common phrase, but you may have heard of it before depending on how much you've delved into certain types of wordplay. Each of the "1"'s in the enumeration is "A".
Hmmm. Each phrase sure has a lot of repeated letters...
What's the most interesting part of a palindrome?

Deacon White

There doesn't appear to be any vowels in these words. Perhaps you should add them back in. The second one should hopefully look a little familiar even without the vowels.
The first one is not the deadly sins, but there's a thematically similar set with a lot of similar elements. The second one is about time. The third one for you is probably right at hand. The fourth one is in the cards. The fifth one deals with a particular fictional family. "VNTRPP" is the last name of the family. The sixth one is from a video game series.
There seem to be some missing elements.
Look at the first letters. Don't forget, this was about adding back in vowels to words where they were missing.

Mike Trout

Each row is a set of three numbers where each number is a 0, 1, or 2. This implies a certain kind of code. Click here for a reference if that's not ringing bells for you.

Mikey “Starz” Yastrzemski

Look at the title of this puzzle. Mikey Yastrzemski doesn't have the nickname "Starz", so we obviously added that for a reason. What's so special about "Mikey Starz" compared to "Yastrzemski"? Do that with these phrases. In most cases, there is only one working word. If there is more than one word, it won't matter for extraction.
You did something with one letter in each case. That letter seems important.

Norman Osborn

These look like trivia questions because they are. Answer the trivia questions. Don't be afraid to search things up on the internet. You're allowed to use whatever resources you like.
Those numbers next to the categories are the indices that you need. The trick is assigning them.
The answer to this puzzle is a word from a work of fiction. You've likely got the second answer wrong. (There's a frequent misconception with that question.)

Pete Rose

The clues at the top of each of the pictures should have a pretty clear answer. (Or at least, the first couple do.) Now compare those words to the pictures underneath them.
Different parts of the picture refer to different letters in the word clued above them. You can use logic to find out which parts of the picture applies to which parts of the word.
The picture in the bottom right doesn't have a clue on top of it. What could be on top of it?

Robert Clement

This is a cryptic crossword. Each of the clues is a cryptic crossword clue.
There is a letter missing in each clue.
There is a letter missing in each clue.
All of the letters that were missing in the clue came from a certain part of the word. You're going to have to do a similar transformation on this word to get an actual word.

Willie Mars

These sound like advertising slogans. Seem if you find some advertising slogans that are similar to these.
Let's take a look at the first slogan. You should have the candy slogan that it is close to. What do you use to clean a floor. Start listing them out. One of them should be similar to one of the words of the candy.
Each of these joke candies is one letter off of the real candy.

Metapuzzle

This puzzle needs the answers to the other nine puzzles in order to solve them. That being said, you'll be able to solve it even without all the answers - you just may need to do some Wheel of Fortuning.
There's an empty column on the table that says "Position". Maybe you should figure out what position that each player is playing.
Good. There is nothing in the answers that tell you that. The puzzles themselves will tell you.
Take a look at Norman Osborn. He's not a baseball player. Who is he? Go research it if you don't know. Once you've done that, his identity should be similar to many of the things in his puzzle. What's a two-word phrase that would be a category that fits Norman Osborn and everything in his puzzle? That has a specific meaning in Baseball - well for one specific team in the Northeastern United States. That item has one position that it's much closer to than others. That's Norman's position. Each of the puzzles has a link to a position like this one does.
How were the letters encoded? (This is a bad pun.)
One baseball position can obliquely describe the transformation that has happened to one word in each of the clues. (This is probably the weakest connection out of all of them, but this position was hard to do.)
Which letters were the most important in this puzzle? What are some ways of describing those? Which of those is also a baseball position?
Each of the items in this puzzle are slogans for candy. Come up with other words for slogan until you get something related to baseball.
What kind of triangles are featured in this puzzle? (In particular, while there are many types of triangles in this puzzle, what kind did you use to solve the puzzle?)
He's not a baseball player. Who is he? Go research it if you don't know. Once you've done that, his identity should be similar to many of the things in his puzzle. What's a two-word phrase that would be a category that fits Norman Osborn and everything in his puzzle? That has a specific meaning in Baseball - well for one specific team in the Northeastern United States. That item has one position that it's much closer to than others. That's Norman's position.
What do each of the pictures feature. Come up with words to describe that until you come up with a baseball term.
You did something to one letter in each row. Come up with words to describe that action until you come up with a baseball term.
Each of the lists was missing one element. Which element was it?
Each position in baseball has a number associated with it.

Answers

Each of the greek letters can be solved for with geometry. This mostly uses the Pythagorean theorem, while there is a bit of basic trig ratios and similar triangles involved as well.
α=18=R
β=5=E
γ=3=C
δ=20=T
ε=1=A
ζ=14=N
η=7=G
θ=12=L
ι=5=E
Each of the phrases clues a palindrome with the enumeration in the numbers on the right hand side. Each of these palindromes has an odd number of letters, which means that the letter in the direct center is used only once. Extract that letter from each answer to give NO NAP ANON.
NEIL, A(N) ALIEN
EVIL (O)LIVE
AIR A(N) ARIA
NOT A BAN(A)NA BATON
SOME MEN INTER(P)RET NINE MEMOS
BOMBARD (A) DRAB MOB
GOD DAM(N) MAD DOG
DO NOT B(O)B TO NOD
A CAR, A MA(N), A MARACA
Each of the columns is a list of ordered items, with the first item and every vowel removed. Figure out what the different lists are, figure out what the first item is, and then take the first letter of each one. All of these are consonants, so add in some vowels to get LISTICLES.
Levels of Hell - LIMBO - L
Days of the week - SUNDAY - S
Fingers - THUMB - T
Henry VIII's wives - CATHERINE OF ARAGON - C
Von Trapp Children - LIESL VON TRAPP - L
Fallout SPECIAL - STRENGTH - S
Count the number of each animal in the mosaic on the second page. There will be either 0, 1 or 2 of each animal. Each row of 3 can be used as a trinary number, which can then be translated into a letter. These letters spell out MENAGERIE.
111 - M
012 - E
112 - N
001 - A
021 - G
012 - E
200 - R
100 - I
012 - E
Each of the sets of words can have one letter duplicated and then anagrammed to give a new word. (This is also the case for "MIKEY STARZ" to "YASTRZEMSKI".) Extract the duplicated letter from each word to give the answer ITCANDLE. Do this one more time with ITCANDLE to give IDENTICAL.
CHAIR LOTS - HISTORICAL - I
HIT SCALE - ATHLETICS - T
PIE CART - PRACTICE - C
NICE RAM - AMERICAN - A
NIUE PALS - PENINSULA - N
NEW DIGS - WEDDINGS - D
GREAT SILO - LEGISLATOR - L
ACT OLDER - RELOCATED - E
Answer each of the trivia questions. Each of the answers fits the enumeration in the parentheses on the right of the question. Once you have all the answers, each category can be assigned to one answer, and the number next to the category is an index into the answer. These indices give SLYTHERIN.
Oscar the Grouch - Muppet (2) - S
Crocodile - Real Life (8) - L
Beast Boy - Comic Book (8) - Y
Cthulhu - Short Story (2) - T
Shrek - Dreamworks (2) - H
Phillie Phanatic - Mascot (7) - E
Greedo - Movie (2) - R
Mike Wazowski - Pixar (12) - I
Kang and Kodos - Animated TV (3) - N
Each picture of a house is a cryptogram for a word that is clued by the phrase above. Solve the clues and use those to discover what each of the parts of the pictures represents. Then, decode the last picture to give the answer MUNICIPAL.

Letters
Chimney - L
Tree - S
Flower - I
Swing Set - N
Round Windows - O
Shutters - C
4 Panel Door - P
Steps - U
Mailbox - M
Doghouse - A

Clued Words
ILLINOIS, MILLION, MOSAIC, ALUMNI, CAMPUS, MUSICAL, COMPLAIN
Each of the clues in the cryptic crossword has one word that is missing its last letter. Take those letters, and you get TERMINAT. This is a word that is missing its last letter to give TERMINATE.
1A. Double Definition - CLIP - Hi(t)
5A. State is RI, Note backwards is AL - RIAL - Not(e)
6A. Sample of bathroom marble is OMMA. Around means to reverse it - AMMO - Revolve(r)
7A. Chief of Billeting is B, appaled at the outset is A, together with BY - BABY - condo(M)
1D. Taxi is a CAB, risk capital is R, CAB limiting R - CRAB - tax(i)
2D. Disseminated mail is an anagram - LIMA - Bea(n)
3D. I'm is IAM, heading to bar is B - IAMB - dram(a)
4D. Primary lesson of youth starts is take the first letters of primary lesson of youth - PLOY - Stun(t)
Each phrase is a slogan for a kind of candy, if that candy had one letter that was different. Take the new letters from each one to spell MOUNDSBAR.
TOOTSIE MOP
TOKE 5
PUP ROCKS
MR. GOONBAR
DOUR PATCH KIDS
SILKY WAY
AIRBEADS
SNACKERS
TRIX
Each of the puzzles can be associated with one of the positions of a baseball team. This association is done via a property of the puzzle as opposed to a property of the answers as is typical for a metapuzzle. Each position of a baseball team can be associated with a number from 1-9. Use that number to index into the answer to get GNOME RUNS.
Mike Trout - Third Base (The numbers were in Base 3) - 5 - MENAGERIE - G
Robert Clement - Shortstop (Each of the clues had one word that stopped short) - 6 - TERMINATE - N
Andy Pettitte - Center Fielder (You took the center letter from each palindrome) - 8 - NO NAP ANON - O
Willie Mars - Pitcher (Slogans are pitches for a product) - 1 - MOUNDS BAR - M
Al Kaline - Right Fielder (The puzzle involved a lot of right triangles) - 9 - RECTANGLE - E
Norman Osborn - Left Fielder (All of the creatures in the puzzle were Green Monsters, and the Green Monster is in left field) - 7 - SLYTHERIN - R
Pete Rose - Catcher (Each of the pictures was of a home) - 2 - MUNICIPAL - U
Mikey "Starz" Yastrzemski - Second Base (One letter was doubled) - 4 - IDENTICAL - N
Deacon White - First Baseman (Extraction involved taking the first letter of the first items in the lists) - 3 - LISTICLES - S