Electrum can import valid BIP39 seed phrases - correct.
Correct.
Electrum does NOT use its own word list, and it has not done since 2015, correct?
It uses the same word-list as BIP39 but also has this feature where user can use a custom word list with an arbitrary number of words in it (in other words it is not strict to have 2048 words).
Also, electrum has its own way of validating a seed phrase, but how?
Electrum uses its own algorithm which can be seen here:
https://github.com/spesmilo/electrum/blob/master/electrum/mnemonic.pyIt's basically generating an entropy then computes its HMACSHA512 of it and if it starts with the desired bits (that indicate type of the addresses to derive from it) it is accepted, otherwise it will be incremented until it is found.
ref.
(1) And once you have a seed phrase, how can you make a master seed from that? With normal BIP39 seed phrases, the process seems pretty clear, but is this Even possible?
(2) And is it possible for a seed phrase to be a valid electrum seed AND BIP39 seed?
(1) Very similar to BIP39 using PBKDF2 but the normalization is different and the salt used is "electrum+pass"
(2) It is possible to have a mnemonic that is a valid Electrum mnemonic and a BIP39 mnemonic but the wallet won't generate such seed phrases because in the process I explained above it will reject those and keep incrementing
refIf you convert your BIP39 phrase to a master seed, you can easily generate other blockchain addresses like LTC etc etc etc, but can you do the same for a electrum generated seed phrase?
Seed is just entropy and entropy can be used to derive child keys and those keys can be converted to keys and addresses of any altcoin.
Since the process seems to be different, is it then in theory possible that the same BIP39 phrase and electrum phrase will get 2 different master seeds?
They WILL get different BIP32 seeds because the process is different. As I said the PBKDF2 uses a different salt so the only way of getting the same seed is to encounter a collision which is so unlikely we can call it impossible.
Another thing.. How are normally electrum wallets imported into other wallets, if the seed phrase is different. Is it just by xprv or wif?
Either the other wallet supports Electrum algorithm so you can import the seed phrase directly, or your only option is to convert it to an extended key (xprv) and import that or import individual keys (WIF) that are both universal and supported by all wallets.