There are no battlefield spells that cure afflictions that I know of. Regen reduces the chance of afflictions, so it could well reduce risk of disease from old age as well, but the old age mechanic could just as easily work on a different principle.
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!