'88 300 SE, Stripped Rocker assy bolt locations in head

[gschira]

1988 300 SE, 240,000 k, Mobil 1

#5 Rocker assy. somehow just pulled loose (as in stripped threads on all 4 mounting bolts) last Tue am when I was arriving at work. Tapping sound and rough running. I thought maybe one or two lifters, or sticking valves, just going by where the sound under the VC was coming from , so when time to leave work drove home (20 miles). Seemed to get up to speed ok, even with the rough running, and was not as loud, so I put off opening up VC until weekend. Put a little Marvel oil in fuel in the meantime and drove the rest of the week. It actually started easier cold or hot than before , which I thought strange. Imagine my surprise to find what I found! Engine has plenty oil, and was not running hot. No extreme accelerations (ever).
What caused the force that stripped those bolts? Would a sticking valve cause this?
Will helicoil work ok in that area?
Does anyone know what size those bolts are or part numbers?
What torque?
Should I change valves? Would they have bent somehow and how do you tell?
So many questions? I have read some of the archived threads, but can't seem to find anything on the CAUSE of this. Is #5 a bad spot in the head? I did read about someone else having same (#5 assy.) problem.
Except for hard starting when hot (normal OT) the engine ran very good with no noises, until this just suddenly happened. Engine VERY clean inside.
Any educated thoughts on this problem? Sorry if I'm not calling parts by proper name. Feel free to correct me as I would like to know..........Thanks