Do you believe in address/private key collision?
It's not something you can choose to believe in or not. It is pure math. According to Pigeonhole Principle when we have 2
256 keys and usually 2
128 hash in the address, there is going to be addresses (pigeonhole) that correspond to more than one key (hold more than one pigeon).
Wallet B of 12 to 24 words with or without passphrase generate the same xpriv or the same addresses?
At the heart of BIP32 as a KDF we are using hashes and hashes can have collision meaning there can be same key being derived from two unequal seeds.
I know it's such an absurd probability that we don't even need to worry about it.
So what exactly is your question here?
Before the seed phrase pattern was so well known, wallets like Bitcoin-Qt (before it was renamed to Core) were known to generate 100 random keys by default, ~ BIP-32 solved this, creating the concept of "seed",
Seed is different from "seed phrase". Seed is the entropy entered into a KDF to derive a "tree of keys". Seed phrase is a set of words that represent that seed.
Bitcoin core still doesn't use any seed phrases (BIP39, etc.) algorithm.
As the algorithms used in this process generate infinite child keys, is it possible to generate all existing private keys in the universe, considering other derivation paths?
Theoretically it should be possible. There may be unforeseen exceptions in the implementations of BIP32 considering it is not designed to derive 2
256 keys (like the depth field that is limited to one byte and is encoded into the base58 extended key).
If you create addresses infinitely, one day you may generate an address with balance, and this address probably belongs to another seed
The universe would end before you can generate that many keys.
And finally, as I was writing another question came to mind, addresses created randomly without seeds, are they possibly associated with a seed?
That's the same question as the collision above. It is theoretically possible for an address generated randomly to also be generated in a deterministic way but the possibility of it is so small it can be considered zero (aka impossible).