My mom puts all her TP in the wastebasket in the bathroom as well. I have no idea why. It's particularly bad when she has diarrhea.
I have long ago given up on getting her to place it in the toilet. She has habits that are so ingrained that she does them without even thinking and no amount of persuading or reminding is going to change that.
My hubby has a toileting bench with high-rise handles on each side. For him to sit backwards on the toilet, he'd have to put his legs through the arms of the bench. Maybe this would work for your mom.
My mom only puts TP in the toilet when I am there to remind her. The other times she either throws it in the trash or outside. I have no idea why she does this and I no longer say anything to her because it is useless. The memory care unit that I have applied to has said that they can get my mom in there within a month. Then they can deal with it. I know it sounds harsh, but I have been taking care of my mom for over 2 years and I just want to do what's best for her.
I find that offering a children's picture book has helped. My client loves to read and although she once devoured books and magazines, she can only process books with a few words. By giving her the book she has a reason to sit and pause long enough to potty.
I have long ago given up on getting her to place it in the toilet. She has habits that are so ingrained that she does them without even thinking and no amount of persuading or reminding is going to change that.