I think you're right, if the fuel pump was bad it wouldn't run well in limp-mode. I'm pretty sure it's either OK or it really doesn't work.

Fuel pressure regulator?? Fuel pressure isn't measured by the ECU, so if it's wrong, the ECU would have to change the injector firing to make the O2 sensor happy. Then the whole system doesn't 'add up' correctly. Code 43 since it doesn't know if the injectors are bad, fuel pressure is wrong, or O2 sensor is wrong.

AutoZone has tools to 'borrow' for free, maybe they have a fuel pressure gauge. Then maybe someone will know what the pressure is supposed to be for 1990??