That's all very well upwind etc. but you'll find that with the kite up, your small amount of weather helm will have swapped to lee helm. If you try and dial this out, you'll have waaayyy too much weather helm upwind (very hard work and slow). Keeping one or both boards down can help to keep the boat tracking downwind even with this lee helm but you need to balance this with the extra drag and the potential for a pitchpole. I found that the Blade tolerates the boards being down very well (they're much shallower than the Stealth boards) and the boat is quite tolerant of burying anyway.

With regard to the mainsheet/backstay thing: that's exactly what the leech of the main is doing, acting as a backstay. Downwind with spin up, you sheet in the main so that it is pretty tight (but not upwind block-to-block tight) and then play the traveller. Don't let the traveller out too far though because although it won't break the mast, it will power up in a gust rather than being stalled and can then promote a bow bury or pitchpole. Keeping the mainsheet on and your mast rotated 90 degs prevents you breaking your mast from the very high point load at the top of the mast generated by the spinnaker.