I had a blown head gasket in my Honda K20, it had absolutely perfect compression. Only way I could tell it had a BHG was that it was slowly eating coolant and the exhaust smelled like it when cold. I still track raced it in this condition!! (and at 255,000 miles, Behold the power of K-series!!!).... LOL. But they CAN be hard to track down, or they can be easy as hell to spot. If your exhaust smells nothing like sweetness, or coolant, then I would say there is probably no coolant mixing with combustion gasses. Have you lost any coolant since you filled it? There is coolant running through the throttle body, but not in a way that it can leak, and you would also smell it if it were coming from anywhere like that.
In every early Maxima I have seen in person (that makes 4) the temp guage sits between 1/8 and 1/4 when it is warmed up and running at the right temp with a good thermostat. If you dont trust your guage, you could use a cheapo analog guage from Autozone.
No, the 280zx came with the 3 speed auto only, which was also a weaker transmission. The 4 speed will haul a boat. In all honesty, This is actually one of the few Autos that I have EVER driven that I didnt mind so much and actually shifted really nicely. Maybe mine was an odd experience, but I liked the L4N transmission. Unlike almost all other Datsun transmissions, the L4N has a locking torque converter Another advantage to the 4 speed is that it has a one piece driveshaft. Which is why I had to switch to 5 speed, the driveshaft was blown and it was either go 5 speed, or have a new driveshaft built for the auto, and I was not about to pay for that.
The cold start injector is in front of the intake manifolt, it has a plug similar to the other injectors on it. Unplug it and see if your start issue changes at all.
I reccomend getting a real factory service manual. It has to be an 83 or 84, because 82's only came in 3 speed auto. Seriously there is so much stuff in that manual that I would be totally oblivious to if I hadnt bought one.