Tendinosis just means there’s chronic/long-term damage to the tendons, caused by a bunch of little tears in the tissue, and from what I understand it heals pretty slowly. I don’t think any of us here are physical therapists or orthopedic doctors, so you can probably get better general medical advice from professionals. My main advice would be: take breaks, don’t work too much, get enough sleep and exercise, eat properly, don’t get too stressed out, etc. As far as keyboard-specific advice goes, I think it sounds like you’re on the right track with the Kinesis Advantage.
I think the main things causing tendinitis and tendinosis while typing are wrists bent upward and excessive force mashing the keys down. But I haven’t seen any especially conclusive studies about this. (They might exist, if someone has some good references, I’d love to see them.)
I wish there were better research about the specific shocks felt by different tissues (bones/cartilage/ligaments/tendons/muscles) on different types of keyswitches and different keyboard shapes/positions, and with different typing styles. None of the papers I’ve seen are actually all that helpful, so take every bit of advice you get with a pinch of salt, and do what’s comfortable for you. This is more folk wisdom than hard science.
With that said, I find the most comfortable typing to be with my upper arms held loosely at my sides (not sticking out sideways or extended forward), my elbows at a 90–100° angle, my wrists as straight as they can get, and the keyboard angled to be parallel to my forearms (i.e. flat or slightly down-angled). On a fully split keyboard, the arms can be put in an even more neutral position by tenting the halves upward toward the center at a 20–40° angle, and turned by whatever amount keeps my wrists straight (the best amount varies between row-staggered vs. column-staggered keyboards). Occasional changes of position (e.g. switching between standing and sitting at my desk) are also helpful.
I’m hardly an expert on this either, but I find I best avoid fatigue when typing by trying to keep my finger motions springy and light. I want to make sure I put enough energy into the keystrokes to reliably actuate the switches, but not smash my fingers down any harder than necessary. There’s nothing wrong with “bottoming out” the switch, but mashing it down really hard into an abrupt stop ends up putting a lot of unnecessary shock into every press. (For me, bad rubber domes cause much faster fatigue than discrete mechanical switches, and I personally appreciate both tactile and audio feedback at actuation. YMMV.) I find it helpful to not rest my arms on any surface while typing, that way I can more easily move my hands slightly around the keyboard, to reduce the amount of reaching my fingers need to do (but without moving my hands so much that they’re way out of position for the next keystroke). Additionally, by having my forearms floating free, my whole forearm can pivot slightly at the elbow. The theory here goes that some of the shock of the keystrokes can then be absorbed in the large muscles in the upper arm which are responsible for flexing and extending the elbow. This is much less important/possible in typing on a standard keyboard than it is on chording stenography keyboards or when playing piano chords, but still helpful I find.
Assuming the upper arms are hanging straight down from the sides, I don’t think it’s impossibly much work for the biceps to hold the forearms up. The biceps are big muscles and should be able to hold the weight of the forearms for a substantial amount of time without getting too tired. In between bursts of typing, a palmrest can be helpful, or the arms can be rested in some other way, e.g. on the lap, the table surface, or chair armrests.
But again, as always, YMMV. If you find it’s more comfortable to use a palmrest while typing, go ahead.
