There's a local gal here in Virginia that has a weekly column on pet behaviors. This is a PDF sheet with tips on why you'd have litter box issues. Maybe a few of them may help. The type of litter is one of them and having to use a box with other cat scents in them may not work as was mentioned. I hope it will help: http://www.centerhillschool.com/litterboxissues.wps.pdf
I sure want you to be able to keep the cat. Sounds like a really good home with how much you've been trying to make this work. Bless you!