Yesterday I found an NX-8V2 panel. The ac mains recognition is different than that of an 8E panel but much simpler. The only pin responsible for recognizing the presence of 16-18 vac from the transformer is pin number 33 of the microprocessor (U6). A 12 K ohm SMD resistor is clamped directly on the right side AC terminal beside the EARTH terminal. The other end of this resistor feeds directly pin 33. So measuring the resistance (without any power on the panel!!!) between the right AC terminal (or the left AC terminal because the transformer's secondary winding's resistance is very low) and pin 33 on U6 it should read 11, 7-12 K ohm. With transformer power on the panel, ac voltage measurement should read 7-9 vac between one AC terminal and pin 33. If your panel does not meet these values check the resistor which is marked R59 on the bottom side of the panel underneath of D3, D5 diodes. Maybe there is a cut on the track from R59 to pin 33 of U6, or R59 makes bad contact on the PCB. The track is very thin.
I simulated your problem as you described it by lifting pin 33 on my panel. I only had one NX-148E connected to the panel. I powered it up connected the battery and …I got the “Control power trouble” message although mains power and transformer where OK. This pin is a digital I/O pin. It only recognizes the presence of a voltage not the value or the quality of it. Although it's a 50-60 Hz ac voltage if you DC measure between any AC terminal and pin 33 you get 5-6 volts DC. That is for the panel to know that the transformer supplies power.
If you find any difficulties I can send you pictures. Just give it a try and let me know.