Your build looks sweet! Way to go getting arou d the issues you have been having with the TTL system! It looks like you solved the PWM throttle reading issue, and the remote not working.
The UPS actually breaks both the positive and the negative of the TTL power because all ESC implement their integrated power switch slightly different. Some of them disconnect the high side and some disconnect the low side. In your case, since the Ubox disconnects the GND, then maybe the high side of the UPS is damaged and stays connected. That would explain the TTL lights turning on due to the UART connection.
Very interesting and good to know! I havent heard that before.
Ive dont this dance so many times! I think if I did another build I would want to try using push button hood latch pins. Those would make enclosure removal super easy 