I have a special needs adult daughter (Williams Syndrome) so I understand your concerns. I have taken her to England every year for most of her life. (she is now turning 30). She walks a little slower and needs the elevator (lift) but will do stairs slowly (two feet on one step) and she will only walk so far! She does not like the escalators, but she has learned to stand on the side, not look down, and is OK if someone holds her arm and is standing behind her. This June she managed London during rush hour. Generally, speaking, I found most people kind on the trains; giving her space to hold on to the pole or giving a seat up for her. At one station, a man kindly took her luggage up a flight of steps. I was really taken aback by the kind gestures I received. Although you can't rely on the kindness of strangers, I will say there are good people out there! She even got stuck in one of the turnstiles with her backpack (my fault I should have let her go first), but as soon as she did three strangers literally came running to help her as I tried to calm her down. We got her free in a few seconds and we were on our way.
If your teen can't manage the underground then take a taxi. It is very easy to do. It will cost more, but you know what your teen can tolerate. If the Elizabeth line is working I think you would be able to take that. Even the more expensive Heathrow Express will be fine. There is an elevator that takes you down to the platform. You can also download the TFL Go App which will give you the routes that are "step-free" It will have a disabled sign on the route.
You can easily book a private car from Heathrow to take you to your hotel. It is not so expensive that it is unaffordable. The airport is west of central London and does take a while to drive in. They will just be waiting in the arrival hall with your name on it.
Just in case this is an issue: There are plenty of bathrooms available once you land and get into the airport. There are some just by the e-gates and again in the arrival hall. You may have to pay at some underground stations to use the toilet.
Happy travels and enjoy London