If I create a wallet I get one seed phrase, one account and one private key.
Nearly. You get one seed phrase, but that can translate into multiple private keys, and therefore multiple accounts.
If I push another private key in my wallet then it has two private keys (two accounts) then the seed phase has to change then I can recover my wallet with two ...
As mentioned in the question you linked, the mnemonic phrase was set to undefined. Another user was likely running into the same issue, causing the same mnemonic phrase to be used multiple times. This was not a collision in a randomly generated mnemonic phrase.
24 word long mnemonic phrases are generated from 256 bits of entropy, which is the same length as ...
HD Wallets refer to Hierarchical Deterministic wallets. These wallets can generate multiple "child" keys from a master seed using a derivation algorithm. The seed is generated from a collection of words, usually 12 or 24.
A keystore file contains an encrypted private key.
MEW Wallet is the mobile application.