elveax:
But, further testing revealed that leaving RST disconnected still resulted in a good boot (RST pulled low did not work).
Indeed I have heard this before: RST can be left floating (it has internal pull-up maybe?), but you have the risk of spontaneous resets. Pulling RST low resets the device, so if you do that of course it won't boot.
This test was with an ESP-12F - tomorrow I get a new set of ESP-12S's in and I'm going to test those to see if they boot different (I think they do, because my previous PCB projected booted fine with 12S's without touching 15 or CH_PD).
That must have been mostly luck. Those two really have to be connected for proper boot.