It is not my view that BOB now never existed. However, if Cooper kept Laura from being killed, presumably Maddy wouldn't have come to town for the funeral, so she would have survived. Two birds?

But Leland killed Teresa a year before that, so she'd still be dead.

Hmm yes I agree on that. And Leland commits suicide a year after Lauras disappearence. 

So he was possesed by BOB and killed Theresa. 

which forces the question, who did Bob inhabit after leland killed himself?

which further begs the question, who did bob inhabit when leland died in season 2?

did he just get sucked back into the Black Lodge?

Presumably. I don't see any evidence that he inhabited anyone in between Leland and Cooper, do you?

And, I guess Cooper still comes to Twin Peaks and ultimately disappears in the version where Laura disappeared...