Hello and thank you for consulting JA,The prone position (on tummy) is very important to the next step in development, namely, getting up on all fours and crawling and getting to sitting. It sounds like she is just on the threshold of this step. Her mind wants to do it and she gets board and frustrated after awhile. SO in creating the environment (montessori philosophy) to move this next step along...help her get on all fours, support her under her tummy, gently rock back and forth and sing to her. You can also place a rolled towel under her chest so that she likes this position longer. It is natural for her to always roll to tummy so don't worry about back time right now. She should also be sitting soon so support her in sitting on the floor so that she can work on her tummy and back muscles and sit alone soon. It is not a good idea to use equipment like a walker. These just serve as a crutch and do not encourage muscle development.
In order to prevent her from rolling during changing and bath, give her a very interesting toy to play with and sing a song during these activities. Let her fall asleep on tummy and then roll her to back. Hope this helps and have fun with your baby,