thanks for everyone's help - I played it safe and arrived 3 hours before my 940 flight.

However I did get the definitive answer to my questions that might help others in the future:

BA do not close their check in desk at lax after the 1540 flight so you can bag drop 5 hours early should you be kicked out of your hotel and on the late flight

You can exit Tom Bradley without having to reenter immigration - there is a glass panel exit to the right of tsa security as you look back on it

Thanks again!