It looks like the first shop you took it to dropped the ball, big time.
Don't know if the M-A-F sensor was actually bad in the first place, but if it was, that
should have cured the problem. And installing plugs&wires, etc. was clearly a case of
throwing money at the problem, instead of actual diagnostics.
Usually if the M-A-F is failing, the ECM will put the engine into 'open loop' (limp mode) so that it can still be driven. Shouldn't just quit running IMO.
Sounds like burned valves indeed, have happened. If the pistons aren't damaged, a new head may be cost effective. But I would look into the first shop paying the bill, since they apparently ****ed up.
This forum may have a higher concentration of members that know these engines really well.
http://pontiacbonnevilleclub.com/forum/index.php