I though of that, but it seems strange it would happen suddenly. It was running fine before the head gasket failed, it ran well for a whole day after the head gasket repair, then the next morning it just wouldn't fire up. That's what has me thinking electrical/timing.

I suppose I could disconnect the pipes to eliminate that possibility.

It did have what sounded like a backfire in the intake once or twice in this "trying to start" saga.

static or mechanical timing?

"plugged exhaust" seems possible i guess. in your first post, you mentioned that you had to rewire the engine harness yourself. i guess that its possible that if the car was being driven around with a bad head gasket and wiring harness, that the cat has now failed/clogged, you might as well rule that out by dropping the exhaust.

if its not a clogged cat....

you mentioned that you have both fuel and spark, do you have air? also check to ensure that there are no obstructions in the air filter box, crossover pipe, ETA. if there are no physical obstructions, check the forum for pictures and adjustment procedure of the throttle linkage(make sure all the springs and levers are present) which is hooked up to the ETA. is the throttle plate in the ETA moving freely when you manually operate the throttle linkage?

turn the key to: key on , engine off position. do you hear the ETA's high pitched hum/buzz? it should be continuous, as long as the key is in this position. in an earlier post , i mentioned that some people on the forum have had to either replace/rewire their ETA's due to crumbling wire insulation. have you inspected the ETA wire harness?
here's a good recent thread on : proper spark plugs(BOSCh F8DC4 COPPER PLUGS), ETA harness repair/syptoms, CPS testing. check it out:
W124 E420 several codes on several pins

if your problem is not related to an air restriction on either the exhaust/intake side, i would have to agree with your post #16, however i would be looking for an electrical problem.

good luck

Well, I found the problem.

First problem was definitely the wire harness, which I rebuilt, but got a no-start condition afterwards.

The answer is - when doing wiring and encountering a problem, don't just test once or twice, test at least 3 times. I had swapped pins 9 and 10 at the 44 pin engine harness connector block. This reversed my wires for the #1 and #2 coils.

Found the error with more continuity testing - started right up after i fixed it.

I feel like a boob - an entire wasted week.