b) If not shouldn't the fuel pressure stay constant (absolute), so that the electronic control takes care of mixture with a fixed fuel pressure and adjusts injector pulse according to manifold pressure?

Thomas,

 You have answered your own question, with a question! Yes most systems would like to see a "absolute" fuel pressure. So if your manifold referenced regulator is hooked up and working what should happen when your MP goes "down" ie produces a vacuum at idle? The fuel pressure must also go down. I don't have Tracy's EMS (yet?) but I believe it has a mapp sensor built in doesn't it? The mapp sensor helps to compensate for WOT at low RPM and changes the pulse width to richen the mix when you are trying to accelerate. Old fasioned carberators used accelerator pumps and "power valves" to do the same thing. (usually not as well)

Bill Jepson