Gun self defense question?

sadly no. the threat is gone. if you can get him to face you again, the threat is renewed and it would be self defense again.

in Texas, you can't shoot him in the back

but Ron White says "you can shoot them in the back of the leg until they turn around. Odds are they're going to get curious. Whose shooting me in the leg i wonder quietly to myself"

I think you're not supposed to shoot anyone, even with a permit, but then what is the point of it.

think of it like this-- The guy is dead, it's your word over his. You can't talk if you're dead

Seems like it should but since you didn't take action during the mugging but more so after the fact they'll probably rule in his favor

Are you still *reasonably* in fear for your life?

If he is running away, then you cannot shoot; the threat has passed.