Concentrating on the working unit, I'm looking for something/anything that happens in the "red-zone" I've highlighted:
1st line is the touch screen.
2 is a spare probe which isn't connected.
3 Video input reset.
4 iThing chip reset.
5 Small board serial transmitter.
6 Large board serial transmitter.
7 Bluetooth Rx (transmitter from main CPU).
8 Bluetooth Tx.
9 Bluetooth reset (from main CPU).
The broken board sees #4 (iThing) never become re-enabled.
The working board has many things that constantly happen (various clocks) but after spending a lot of time testing different things (on both sides of the board) I can't find anything which changes in the red zone other than Bluetooth's 3 identified lines.
I've included the touch screen because it's the closest thing that "happens" but zooming in, it shows the large board talking to the small board before that occurs. It's also well after the iThing reset is released (goes high).
Every other signal of interest I noted from the working unit is doing the same on the broken unit. Clocks and all the other signals from the various subsystems. Interestingly, the LCD screen & backlight power chips don't get enabled until about 24 seconds in (and when they do, it's for a very short period).
I don't think the main CPU has a broken I/O pin - at least, I can't find any candidates to test. I'm going to go back and re-test all the power regulators, I think, over this entire boot period. Not tonight though.