1. I would not modify the cone of your piezo tweeters by doping them. It's difficult to apply a coat of some form of doping material in a consistent fashion.2. Damping the frames of both motors to reduce vibration couldn't hurt.
3. Additional bracing never hurts. I don't think you'll find any benefit in adding it here, but it never hurts.
4. I've tried lots of stuffing materials and always come back to R13. The biggest thing is spacing away from the boundary, so thickness is important.
5. Using T-nuts is always a good idea. If you don't plan to re-enter the cabinet very often, wood screws are fine for lightweight components. But the more times you tighten and loosen a wood screw, the more weakening stress and wear is applied to the threads. T-Nuts and metal screws are much more durable.
6. You can radius the baffle cutouts if you want to, but I'd consider this a matter of aesthetics more than acoustics.
7. As for spikes, some people like them, others don't. I think it depends as much on your room, floor and floor covering as anything else.
Extra gaskets aren't needed because they come with the speakers. You can seal them with silicon if you wish, but I wouldn't unless you found them to be leaking. If you cut round holes as indicated in the plans and your baffle surface is flat, no additional gasket material is necessary.