Would Lash changing teams have any effect on her and/or Harry’s ability to access hellfire.
This could poke a hole in your theory, place a time for her rebellion, or be completely meaningless, depending on the answer.   

I would say a definite maybe. This wouldn't do anything/ maybe actually support the theory though. Lash didn't switch and start using soulfire until the last few seconds in Harry. That means we never got to see directly if it did anything. If Uriel actually did show Harry how to use soulfire through residual energy from Lash, then her switching would probably mean he couldn't use hellfire anymore (no one would need to show him how, but he still couldn't) I would say that this point makes a good argument for her switching on its own.