Rear seat will not tumble

So you can fold it but the latch release doesn't work? That sounds like the internal release cable in the seat bottom may be broken. About the only thing I can think of would be to see if you can get a grip on the cable itself, it should be on the passenger side of the seat inside the area where the folding hinge is. Grab it with a pair of pliers and see if you can get it to release. Then you can get to the bottom of the seat and replace it.

This shows the cable ends.
https://www.youtube.com/watch?v=2ubzM21pcjc