You would however clearly feel the vibration
Not necessarily. We found one cracked cylinder and one with a broken piston ring on the following symptoms
Increased oil consumption. Not dramatically so, about 60 percent up on the previous 50 hours, and a slightly greasy fuselage behind the engine.
Slight white marking on the cracked cylinder from gas escape.
The cracked cylinder was well down on compression, the other down enough to make it worth changing at the same time. (both on the same side)
No vibration, static rpm 2350 (expected rpm for that engine/prop combination)
Lycomings (and Continentals) are pretty agricultural engines.
There are two mags in order to provide a full burn, not as a redundancy / choice exercise
Well, not entirely. The general idea is to be able to continue flight to a suitable landing place if one plug is out, or of course one magneto. My car seems to manage an efficient burn just fine on one plug per pot, placed rather more centrally. Not choice, but redundancy, yes. I have done a fairish bit of flying on single ignition engines, but always in stuff that will either glide rather well, or alternatively is so slow that an average sized carpark would suffice for a forced landing. I like two magnetos, thank you.