First, if you have confirmed that your new / rebuilt injectors and the cold start injector are not leaking, then I would not get into a sweat about a gradual loss of fuel pressure when parked. If the injectors are not leaking then pressure is being lost through the return line on the fuel pressure regulator or the check valve on the discharge of the fuel pump is allowing gradual leak back and loss of pressure. A leaky check valve is the most likely cause. No leaking injectors and the fuel pressure is maintained at 28-30 psi when the pump is running - you should be good to go. The only problem with the leaky check valve / leaky fuel regulator is that when you first turn the key to the run position you have to allow the fuel pump to go through its prime cycle to pressurize the system before engaging the starter. The prime cycle pressurizes the fuel system for faster start-up.

As an aside, Summit Racing sells Beck Arnley 158-0438 fuel injectors for around $50 which are an exact drop in replacement for the Bosch 036 injector. I have flow tested both the Beck and the Bosch and they are exact performance duplicates. The BA work just fine in the B20 E/F.

I checked the Volvo service manual and at 20C the coolant temperature resistance is around 2000 ohms. Your value of 1950 ohms is likely within spec as there is a fair tolerance on the values.

The D jet system absolutely requires that the cold start injector operate in order to get the engine started. If the cold start injector is non operational the engine will never start. I would have to go back through the control unit schematic diagrams to figure out whether the port injectors even deliver fuel at cranking speed (about 150 RPM). Even if they did, it would not be enough fuel to get the engine started, especially an engine at ambient temperature. On the early Djet, the cold start injector only operates when the starter motor is engaged. I don't know whether the '73 was still set up like that. As a general comment, as long as the control unit is causing the cold start injector to operate during cranking of the engine, the rest of the D jet can be dead and the engine will typically start up while cranking. If the rest of the D jet is dead the engine will promptly die when you release the starter because the port injectors are supposed to take over from the cold start injector once the engine catches. If the cold start injector is operational and you have reasonably correctly timed spark, the engine should catch during the cranking cycle.

The D jet is reasonably reliable. Its probably more reliable than a similar vintage emission equipped Stromberg carb. What most commonly messes the system up is poor electrical connections. The second most common problem is people who park them for a few years and then try to get them to start when they are full of rank gasoline followed by people who try to set the idle speed like they do on a carburettor. The D jet is a poor choice for people who don't like to read service manuals and whose favorite tools are a hammer and chisel.