Ok, I'll throw my hat in to get things started.

First, these are very good questions. Second, I don't know all the answers.

I think a lot of the planform shape design comes down to style and manufacturing techniques. If you have CNC you can get as fancy as you want (Avionik) but if you are using hand made plugs for your molds, then simple straight-tapers will be what you see (like the one I'm building right now).

I haven't done any math to prove it, but I think the elliptical shape also increases stiffness a bit, thus minimizing the structural material (weight) required.

The wing pitching moments and the required max Cl for the stab are pretty small (and I've never seen one stall) so the symmetrical sections seem to do ok. I guess there's a tradeoff between drag sources... the profile alone vs trim drag, etc.

I think the Avionik D99 stab is a little too small (forcing a forward CG) but that's just my opinion from flying them.

Other ideas??
-David