The reason you need power in the NGX from what I have tested is the outflow valve needs to be opened and in default it is closed and in some magical way the cabin is pressurised until it opens, once the cabin pressure drops below 0.5psi DP the doors can be opened.

I have seen that all doors but FWD L door can be opened with any power from FMC, even air stairs. Maybe the door should be opened without power but not the stairs as they need hyd power or electrical. I will send a ticket to PMDG to ask about being a bug or not. Xiropillo