In that case, you have not properly configured the URI match detection.
The latter former.
Yes. Using a random 4-word passphrase means that an attacker would, on average, need to test over a quadrillion guesses, on average, before hitting on the correct master password in a brute-force attack. Bitwarden’s default KDF settings are designed to throttle the guessing rate to 10,000 guesses/second per GPU. Thus, 6,000 GPU-years (and millions of dollars) would be required to crack a Bitwarden master password consisting of a 4-word passphrase.
I think you’re referring to a BIP39-enocding of a 128-bit or 256-bit cryptographic key. Different application.
No, you will just be offered both vault items as autofill options.