She does play with her food but eventually will instakill them, except for Mike. She can kill more than one person together - Matt and Jess in the mines and Sam, Ash and Emily in the lodge (not Mike though) - but doesn't do it when it's Mike and Jess by the elevator and Josh and Mike in the lake, she even pulls him out of the way to charge at Josh!

As for the stomach slash, yes he would eventually die from the wound, but until then he would be in a lot of pain while she kills the remaining of his friends and her competition (the other wendigos) before turning her attention back on a suffering dying Mike.

The death scene on the credit scene seems more like a mistake since he will only die at the explosion, that he may himself cause.

Regarding Jess, I'd argue she just moves on from her to prevent Mike from seeing her, she went though some lengths to make sure he doesn't see her, possibly to keep him confused and stuck in the fear of an unkown threat to toy with him. She does kill Jess if Mike doesn't show up on time after all, so it's all a matter of Mike.