I have a feeling you don't work in entertainment whatsoever. If she got her doing this stunt, she couldn't tour for months, she has to dance and be physical as much as her voice is on show. They wouldn't risk the millions it would cost if she were hurt and couldn't tour. Sorry, but you're completely naive. And because there are obvious queues on where she is swapped out for a stuns double or green screen, makes it even move clear she didn't do the stunt. She wouldn't risk not being able to tour for a "75,000" commercial, which also, it was not, I'm sure they spent at least 3-4 thousand on it. I've worked in film, highly doubtful they skimped on this.
Sorry, but everything you wrote is completely false. It wouldn't take "months" to heal from anything she might theoretically break. Besides, she did the Apple commercial very recently, and her 1989 tour is over. TS has no upcoming albums to promote and no scheduled concerts.
Dancers learn how to land or fall without injuring themselves. She was probably also wearing pads underneath her clothes. Even the treadmill was likely padded, and the tread itself is just rubber. It's even possible the treadmill is CGI. With careful choreography, and a team of people helping her figure out just the right angle to shoot from, there is absolutely no reason why she couldn't have done the pratfall herself.