No, they are not all built on long actions. You're being confused by Savage's switching back to identifying all new models Model 110's whether they are a short or long action. No different than the Rem 700's which have long and short but use the same model number.

6.5 CM on a long action - I can't see why one would need to, but no reason you couldn't. Biggest issue might be getting it to feed properly from a L/A magazine depending on how long the feed lips are.

Yes the Stealth chassis (MDT LSS chassis) symmetrical. It is machined as ambidextrous and just has a plastic filler piece in the bolt slot on the side that isn't being used. Just have to pull it out and put it in the other side IIRC.