Hello Tom,
1) If arduino nano is running on 12V, but not via usb (and if usb connection is OK.) -? seems like something happened to arduino nano (power circuit?? MS1117? diodes?). Do You have USB power measure tool like Keweisi? So, You could measure (if any) current consumption by arduino nano via USB. Anyway, if nano is NOK it is not expensive to buy, reflash and replace.
2) Low RX might also be caused by Q10 transistor failure (and next ones Q11, Q12.. in RX chain). Please check voltages on them.
Regards
Tomasz
tomekdm sp7etd