1. I still prefer domes, but quite honestly the only choice you have is the Dayton Neo20. If you want to go more expensive, then there are simply no other 3/4 inch neo domes that can have their flanges cut tight enough to make the c-to-c requirements, and none of the non-neos will have a magnet structure strong enough to be small enough.
In groups of 30, though, they are simply amazing.If I can't use domes then the B&G Neo is the only thing I would touch.
2. If you want a 3 inch midrange, since the SAMMI's are no longer available I don't think anyone has a choice other than the HiVi M3N 3in aluminum/magnesium which has an xmax of 3.0 mm and 90-8k.
If you go up to 4 inch, then I would go with the Dayton RS 100, 4 ohm, hands down.
3. Electronic crossovers can be digital or analog. Unless you need to tailor your system in unusual ways such as a 48db or 96 db crossover, you can get by with the workhorse Rane models. I think the cylindrical wave of the nearfield(provided you are in the nearfield) leaves the need for room corrections in the dust.
4. Tri amping is easy to do. My personal recommendation is, despite the efficiency of the array allowing smaller amps, to have the reserve you might need, and not to go for less that 60 watts rms/ch for the tweeters, 150 watts rms/ch, for the mids, and assuming that you are using a pair of hot 12 inch woofers for the bass, not less that 350 rms watts/ch for them.
Marlboro