However, i have seen that one is allowed to have more than one 2fa attached to an account. Can i attach fresh 2fa (i.e Last Pass Authenticator?) and enter the private key that way?
Yes, and it should be set in a login state whereas you are currently unable to login to that account due to missing 2fa (I read your old thread). I am not a user of both exchanges, generally you will be asked to do KYC verification to delete 2fa. No other way.
I'm just curious about those 12 and 24 word phrases and 2 exchanges, if it's kept by your uncle it should be the key to access or recover something. You don't have to give it to anyone else. It is enough to search for information on each exchange's help, FAQ or suppport pages with the keyword "phrase 12 or 24". If it's a recovery code for 2fa, I'm not sure it can be used in any 2fa app. Try downloading some 2fa apps like G-auth, authy, aegis, or others. Maybe it will work for one of them.