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I have a '91 Reatta. I've gone through 3 oil pressure sending units in the 10 years I've owned it. They are notorious for failing due to crappy quality. BUT, I've never encountered any of the symptoms you are describing due to a bad oil pressure sending unit. When they go bad, the oil pressure dash gauge just goes "haywire", from extreme low to extreme hi back and forth. At no time does it adversely affect any other engine function. When the dash gauge goes "haywire", I know it's time to replace the unit.... nothing more, nothing less. Just my own experience, for what it's worth.

This is a 1991 and the regulator is a very different style. The oem is Bosch, however, it may be possible it doesn't close the return. Only two things, aside from the dash cluster itself, are involved with the tachometer, and that is the ICM and ECM/PCM. A failure of the dash cluster will not affect the running of the engine. It is a symptom, not a cause. As mentioned more than once; hotwire the fuel pump, preferably with a gauge attached. If the pressure is stable somewhere north of 40psi, try to start the engine. That takes the oil pressure switch, the relay and other things out of play. It should start and run if fuel is the main issue. The fuel pressure should drop a little with the engine running, mid 30's is fine, any lower and it is a problem. If it still acts up, the problem is likely elsewhere.

Again what happens if you apply 12v to the loose fuel prime (green) connector and try to start ?