Encouraging or requiring a kid to pray for 40 minutes to an hour a day, removing the kneeling issue from the equation, is not abusive. It may not be something a kid likes to do for that long, especially at age 8, but it is not horrible.
The physical posture of kneeling without leaning for 40 minutes to an hour seems to be the issue. My mother would not have expected me to kneel that long, nor would she have done it herself (physically I doubt that either she or my dad could have knelt for that amount of time). I know there are other cultures and other families that have an expectation that kids will learn to kneel up straight on the floor and this is considered a self-discipline and a learned art. The thing is, if you were doing this every day, it would have ceased to be painful as your body got used to it and become a habit. They did used to teach kids this in the old days of the 1800s and early 1900s in some families and schools, from what I have read.
I think you would be better off discussing this with your mother or with a counselor than with an internet forum, frankly. Your thread title is misleading: you do not want to have a discussion about “Forced Family Prayer Pros and Cons”, you want us to agree with you as an adult speaking right now that your mom was a child abuser. We weren’t there, don’t know the situation, don’t know you, don’t know enough to judge, all we have is your say-so now. I agree it sounds like something else was going on in the family than your mom simply forcing you to kneel and pray for 40 minutes a day.