All things being stock, the heads come right off with engine in frame.
Looks can be deceiving..... head bolts are just a regular 1/2 inch 12 point socket

The head and barrel are both retained by the same stud(as we all know).
I would always recommend replacing the base gasket whenever the head has to come off.
A new set of rings(provided the bore looks good) is a good idea while down.

It is VERY(as in most) likely that the clip came out due to a bottom end failure.
The crank pin is very likely to be heavily grooved.
It almost certainly has nothing to do with previous work done, as the wear may not have even been evident then.
I would not be abusing the previous mech, expecting a refund, or reassembling as is.
It is a very easy job to drop the valves out, and clean up that area. I would not be leaving them.
Remember............ POOR MAN PAYS TWICE!!!!!!!!!!!!!!!!
Even if it was just a bad circlip install, that gunk will be all through the oil pump, cam bearing, etc, etc.
You are very likely to send good money after bad.