Too many to mention.

Since the reset line to the U9 CPU (pin 40) is being released and going high, the valid power detector circuit is doing something. If you very briefly reactivate the reset by quickly shorting pin 39 to pin 40 at the CPU and the LED stays stuck on, your problem isn't the valid power detector circuit.

Is the CPU getting pulsing clock signals on pins 3 and 37 (produced by U16 and U15)? No clocks into the CPU means it's effectively brain dead and cannot execute anything.
If yes, is there any activity on the address/data/select lines at the U6 boot ROM?

You're getting nothing on address line A4 (pin 4) at the U6 ROM, meanwhile address line A4 at the CPU is active (H/L-P). Check for an open circuit on that A4 signal between pin 13 of the CPU and the U6 ROM at pin 4.

It shouldn't. VMA should go low during the CPU cycles where it's executing code internally (not talking to other onboard chips) and between address bus changes when they're in flux.

Constant LED flashing points to an issue with the 6810 RAM chip if memory serves me right.

Oh, you were running Leons ROM which was making the LED continuously flash. Got it.