1- Metamask supports coins of a certain standard (erc-20). If a user has coins of other standards in a hidden wallet (for example, bitcoin), then he will not be able to access this. How to be in this situation?
I have no idea about Metamask specifically as I have never used it, but each hidden wallet can store any number of different coins. You just need a compatible wallet in order to access each one.
2- In what other applications can restore a hidden wallet? Is this possible in electrum? I already know about trezor and metamask (information from the official site). If trezor suite stops functioning (don't even ask for what reason).
Yes, Electrum supports passphrases. Most good wallets do. You will never run in to a problem of not having software which can recover hidden wallets.
I would also just add that Trezor now support government surveillance, and so is a very poor choice of hardware wallet.