I bet when your mechanic tells you the problem, you're gonna kick yourself. Hopefully, not to the chagrin of your wallet. That "small pipe near the front of the plenum running down to the injector rail" I believe to be the vacuum to the FPR.
I think part of the key to your problem is that "The plenum has blown several times". Now you're having no-start issues. It would seem that you've definitely eliminated
fuel starvation as a problem. Now you just need spark - at the right time. You'll figure it out. I've been told (on my Volvo 164E and the Alfa's) not to check spark by grounding plugs to the engine (supposedly might toast your ECU). But I
have pulled the plug boot back, and cupping one hand and looking in the well, see
& hear the plug firing (or not).
If I were you, I'd put a cover on it, & exercise my right to smoke cigarettes and drink pints in a nice warm pub.
...'least till the temps break 50*
