maybe you could check fo r a pulse on the sensor wires, a scope or soemone here had a good idea, connect a radios input and listen for it. ( poor man's audio osciloscope)
you might connect up a few test lamps , ECU power maybe or fuel pump power, "power feed to dme relay?
maybe node lights between the injectors and their input cables. I think if you see in real time by using some temoorarily connected small bulbs, you can then tell when things are on or if power is missing. you cna check with a meter of course but then you go use the meter for somethign else. with small bulbs you can monitor 10 power locations in real time , low cost in that.
I know in my volvo if I pull the fuel rail to try to watch the injectors spray, the injectors wotn fire if I dont reconnec the ground to the engine. someone here might have a source for "noid lights" if you plug them in betweent he injectors and wires you can see them flash as the injectors fire. in real time.
youcan go connecting small bulbs to questionable relays and such to study them for intermittent issues etc. what that can do is just dumb things down a little so you can see visually where the power is or is not.
the battery negative lead is important and ground to engine, almost everything electrical relies on a good return path. You obviously know that if you are this far in, you obviously have good electrical troubleshooting skills.
Id lift and celan every single ground you can find or come across with the moisture especially thoise bad grounds can hide themselves, and poor grounds can cause intermittents and hard to troubleshoot issues.
Ill clean and I use "penetrox A" its a grey grease like substance, if it see it near a ground teminal, I know Ive already cleaned that ground connection , if not I clean and then apply it. If I see no greay stuff it still needs doing.
https://sds.chemtel.net/docs/Rexel%20Holdings%20USA-0004804/FCI%20USA%20Incorporated%20Burndy%20Electrical_Pen etrol-a%20Oxide%20Inhibiting%20Compound_Unknown_01-17-2002_English.pdf
you mientioned the engine sensors were attreacting metal, I guess being somewhat maggetic , they are coils essentially, could there be more debris in there?
in some 944's the clutch center is rubber and can dissintegrate evidently a sign is rubber debris in around the starter motors gear, maybe if you pulled the starter you could see in ther better? clean and inspect? it definitely won't start if you loose the timing pulse(s) the tach blip is a sign.
sounds liks a frustrating project, you may be up against several reasons for a no start due to people mucking about before you. Vey frustrating.. I hope you are successful. Just taking this on is admirable.