This really, really depends on how the game is designed. If swimming is very integral, and often comes up, and the designers are too lazy to give the dungeon multiple pathways while maintaining good level design, then it should probably be possible. I voted only at master, though, it doesn't really matter: how does it make sense to swim with heavy armor in your pack, but not wearing it? Should probably be based on encumbrance vs buoyancy instead.

Big fatigue loss with running and jumping: hell yea.

Rolls and flips: hell nay (with out without heavy armor-- what is this TWitcher?).

Burden at master level: I voted no, but heavily dependent on the game itself. Maybe partially.

Penalty to sneaking: Yes. The immersionistas, realtatos, hardcorporals, and rollerplasters should all be in agreement over this one.

Maybe a bit of mail here and there (goussets), but not a full hauberk underneath! Also, with techniques of the time period, a well crafted suite could weight just 45lbs. But, remember, truly full plate as we're envisioning it wasn't widespread until the 1400's.

Maybe the chain mail weighs less, but the weight isn't distributed evenly so it is more encumbering (particularly a full length hauberk, not just a mail shirt or mail plus other stuff). I have a stainless steel mail shirt, and after wearing it for a few hours, it feels pretty heavy, especially on my shoulders.

I hope they don't keep making the more protective armors weigh more and more. It's just inane, and non-sensical.