Updates on the tendonitis situation, and strings, with regard to the 9-string fanned fret:
I have had to experiment with a practice schedule that allows me to keep gigging. So I rest Monday and Tuesday, warm up on Wednesday, and then play four days. The elbow pain has diminished. Lots of stretching and calisthenics using the reverse movements of the muscles during the off days - if I just rest, then the muscles just tighten up into knots, so keeping them moving is good, I just don't play guitar. Of course this is a slower process than the obvious answer of just resting six months. But it seems manageable. It's just that on first receiving the guitar, I went gangbusters practicing instead of easing into it more slowly, so I worked myself into a bind. But it's not the first time and the recovery process appears manageable. Basically, to fully exploit the fanned frets has required an extended range of motion with the left elbow and wrist, not something to be done overnight.
The lighter gauge strings actually make the guitar sound better. Yes, when playing without amplification, it seems that there is a lack of projection because of previous expectations, but the sound is sweeter and I have noticed a curious phenomenon: The relationship between the sound playing near the bridge and the sound from playing over the soundhole has changed. The near-the-bridge sound is itself much sweeter, and the variations in the tone are more dramatic and expressive with a shorter lateral movement of the hand. This may be a function of the guitar opening up. However, I think this phenomenon is unique to good spruce tops, because I actually have never experienced this with a cedar top.
I am using a .021" D'Addario rectified nylon A4 string. I like the warmth and presence of the rectified nylon more than clear nylon, and the way that it is not as slippery under the fingers. Obviously this is far lower tension than the strings other posters have reported using - .028" has been frequently mentioned. I have had to adjust my whole playing style to play more softly in order to allow the A4 string room to sing. However, the results are far sweeter than I was getting with my experiments with high tension strings on the earlier prototype, and I can get a nice vibrato, which was pretty difficult with the higher tensions. I do have to be careful to play the A4 string a little bit louder when playing full chords and back off on the other strings - this is a technique easily acquired. We did an acoustic living-room gig the other night, and since I have been playing mostly amplified lately except in my studio, I felt at first that I lacked projection. But shortly my ears adjusted and as the audience was quiet and respectful, the sound was at least completely adequate for the space. I have no plans to play unamplified in 4000-seat halls, and for the ongoing development of my own musical purposes and to save my left arm in the process, it is a very interesting experience to be moving away from the concert hall sound and toward more of a chamber sound.