Regen reduces chances of battle afflictions, not old age aff IIRC.
Death reduces chances of getting disease due to old age (30% per skill level IIRC, thus level 4 death mages can only get diseased due to old age if they first get feebleminded or mute).
It could be. I think skinshifters had a different maxage than normal humans, but I'd need to check. If so, it would probably act like 1 level of nature magic.
Before anyone asks, if it's true, NO, the DB maxage calculation is NOT going to be updated to reflect that!