If the noise is induced in the pickups - eg from your monitor screen or fluorescent lighting, then no amount of electrical isolation will cure it.
The only cure is switching off the offending sources, or moving away from them.
A long-shot 3rd option is to get an old Line6 Variax, which doesn't use magnetic pickups at all & is therefore induced-voltage-...
You don't have a ground loop as there is no connection between the guitar and the computer chassis (other than via the interface).
You can try using a DI box - this might improve your situation but without having access to the setup it is hard to diagnose correctly.
Yes sortof maybe.
Special plug ins can help a lot depending on the exact problem.
The only correct thing to do is to get a better recording.
Teach the speaker/vocalist how to use the mike to avoid breath sounds.
Use a quiet room to avoid ancillary sounds.
Noise reduction works quite well to remove hissy type noise but other sounds, NOT properly called ...