Clutch Issues

[Aedan Graves]

Hello all, I'm back with another question. As it is now spring, I've been out working on my bike. Doing wiring, cleaning it up, the works of restoring an old bike. I noticed my clutch wouldn't disengage, I don't recall it doing that when I changed the oil then let it sit for the winter, So it's likely it could have just seized up sitting there for 3+ months. But after doing clutch cable adjustment I noticed that the wheel will turn (in gear) easier while holding the clutch leaver than it would with the clutch leaver released, almost as if its slipping, but is still very difficult to turn.
I am currently trying this method to un-seize the clutch as I didn't see much harm in trying it.

But it still leaves be with a few questions;

1. Will a seized clutch do what I described above, it sort of works? or is it more likely to be a grooved clutch basket
2. Say it is a seized clutch and the linked method doesn't work. Should I go as far as to replace the entire clutch pack? or just manually separate the plates
3. What can I do to avoid my clutch from sticking again?

Thanks, I know I've done nothing but ask questions on this form, but I do hope to contribute in some other way eventually.