I will let someone confirm this. Thank you so much again for the help! It really is appreciated!
Yeah you are correct.
512 bit seed from BIP39 PBKDF2 is not the Master Private Key.
It is an intermediate value that serves as input to BIP32 HMAC-SHA512 function, which then generates the Master Private Key and Master Chain Code.
The Master Private Key is specifically the left 256 bits of the BIP32 HMaC SHA512 output
It is the "mnemonic seed phrase" that is passed into the PBKDF2 function and not the actual binary bits
No like you assumed The seedphrase is not passed directly into PBKDF2 as a string of words. Instead, the mnemonic phrase is first converted back to its binary bits using the BIP39 wordlist not by the function but wallet software.
So the password input to PBKDF2 is this binary string i.e entropy and checksum, not the text of the mnemonic words themselves.