yeah, I'll put in my 2 cents again...
... I honestly don't know what to say about tendonitis and the effect that certain things have on some people. I honestly think that heredity plays a part in that sort of thing but hey, I ain't no doctor.
Just talking from personal experience, I've pushed my fingers, arms and muscles to the limits for years and never had any negative effects. The stretching exercise above for instance was impossible for me to do when I first tried it. After a couple of months I found it playable but still tough.
After a few years it was no problem
I practiced it until it hurt, but was always careful to give myself time to recover between sessions.

There are definitely some things that you can improve on by doing lots of practice. Stretching further is one of them. I mean, just compare your ability in your first year of playing to that of your 15th year. The thing to remember though is that it took 15 years to get there.
Same thing with stretching. There's no quick and easy fix to anything so as long as you stay aware of any unusual pain and take things gradually, you'll be able to add a little distance to your stretch. (a little)

If you're having trouble reaching certain notes or doing certain things, the first thing I'd do is try tapping it. 9 times out of 10 you can find a way to do it more easily than playing it normally.
The exercise above for instance is on my lesson page but I've got the identical run done with tapping and there's no contest... tapping wins every time on accuracy, cleanliness, ease of play and speed.