The trick of falling out of a wheelchair safely is to fall slowly. I learned that in wheelchair tennis.
I played a woman who regularly fell out of her wheelchair, in a kind of tuck and roll stance. She always landed well, bounced back into her wheelchair and continued playing without much of an interruption. I saw wheelchairs turn over, placing people in a position of a turtle with a hardshell on their back. They came up swinging as soon as their chair was turned over.
So I've learned not to be afraid of taking a few falls out of a manual chair. Yesterday, after duct taping the sling on the manual chair, I fell twice out of it. I was able to fall slowly, so I'm not injured, just in a bit of pain. So I had to get into the power chair and stay there without doing any pushing at all. Knowing that will lead to the loss of some of my strength, I ordered the equivalent of a rental wheelchair to give me something to push. It's heavy, but it was inexpensive and at least I can push it a little to try to maintain what I have worked to keep.
Anyhow, here's a song for all those who fall slowly, no matter where, no matter when, no matter how. The important thing is always to keep your eye on the ball.