Thanks for writing in with your questions. Bummer about the knees. I too had to give up running (and many other things I loved to do) due to a very bad injury that resulted in 7 surgeries to my R knee over 35 years. Finally I had the knee replaced at 61 years old (4.5 years ago). Within 2 years and since then I’ve been running 30-40 miles a week no problem. Just had the 4 year check up by the surgeon with x-rays showing no wear in the joint at all. Looks like the day he put it in there. That new knee gave me back a lot of the joy of climbing, skiing, running that I had lost. Don’t close the door on you knee.
Enough of my proselytizing…….On to your questions.
It seems that there may be some terminology confusion. So let’s start by clarifying that:
You note that you AnT is when you move from nose to mouth breathing. That’s not the way we or most people would define the AnT. Please read this article about how to find AnT. https://uphillathlete.com/diy-anaerobic-test/. As you can see AnT is a hard effort that will surely having you breathing deeply and quite rapidly through your mouth.
On the other hand one definition of the AeT is based off of ventilation and often corresponds to the point where you must begin to breath through your mouth or where steady conversation has to give way to breathing (conversational pace/full sentences) and you can only speak in short phrases. Training Peaks offers us another way (outside of a lab test) to determine AeT https://uphillathlete.com/heart-rate-drift/ that you can do on your treadmill.
I would not rely on subtracting 10-15 beats from the Anaerobic Threshold to determine the AeT. Instead try to determine these two significant metabolic events as well as you are able using the several methods discussed on this site.
You will need to do a different tests for cycling and for hiking as they will surely give you different HR ranges.
I hope this explanation clarifies things for you.