A simple compression test will tell you if you have bad rings or a cracked piston (doubtful, both of them).
I vote for a worn valve guide if you have good compresssion. That would cause oil to possibly get in there and such issues with M112s are common.
No need to tear the heads off, you can do a compression test yourself with a test kit, probably can rent the kit from a local store.

I'd not tear into the engine or change it until I KNOW what is going on. The miss can be unrelated to oil on a plug, there are a lot of things that can cause a miss to "come and go".

Oil does NOT evaporate.