It would be surprising to find a MkII Ross supplied to the USAicon that did not have a reamed oversize chamber. Some folks did address this later by setting the barrel back and re-cutting the chamber.

Regarding your questions:
1. If the rifle is working well and is reasonably clean, there is perhaps no good reason for taking the bolt apart.
2. I have seen a MkII that was the victim of a concerted effort to blow it up by plugging the bore etc. In the end they cracked the side wall on the receiver, but it did not let go. Condition of the bore is probably your first concern: obstructions, ringing, anything which could become a safety issue via a lodged bullet jacket or some such problem.
3. I would wipe the stock down with soap and water and re-oil when dry, but no steel wooling, sanding or stripping. The patina is irreplaceable and like furniture, once lost a lot of the value is lost.
4. Sling I'm not sure about, but either leather or perhaps one of the No-Buckle variety. I believe repros of the leather sling are made by Moore Leather