@LoyceV
Thank you for your answers.

RE Q1:
I understand the public key will be available on the blockchain if transactions are already done, but how does the attacker know which transaction? The public key of a seed differs if it's part of a multi sig wallet or a single sig wallet right?
RE Q2:
Safer from an attacker
RE Q4: That's also an option, but passphrases can be forgotten. I'm not saying this setup is a good idea, I just want to know the details and make a good decision.

@alexeyneu
I'll read that stack exchange page. Thanks.

@hosseinimr93
This is for a wallet with less funds and I want it to be able to manage it with max 2 wallets.

@apogio
Thanks for the link.

@alexeyneu
It's not a given attackers know how much you have.

@nc50lc
"A client/wallet will not be able to sign by having only the private key of a single signer without the redeem script or cosigner's public key to reproduce it."
Clear! It's essential to store the public key of each cosigner. If they are not leaked, the private key can't move funds.