It's not the voltage regulator - I just grabbed my 9v and used it to power on a very similar project with the same voltage regulator, no problem. The only difference I can spot between the two projects is that one uses an ESP-12S and the other uses an ESP-12F. That's very frustrating because all the documentation I can find online says that those two modules are more or less identical.
This other project doesn't have GPIO2, 0, 15, RST, or CH_PD pulled in any particular direction...it uses the TC1262 voltage regular and powers with a 9v. Boots fine and flashes. Then I have this new project - same setup, won't boot no matter what I do.