Comments on your points first:

1. Yes on the 8 Fountek NeoCD2.0.
2. Essentially yes. With the Neo2.0 the crossover can be low enough so that trimming flanges would not be necessary.
3. You don't necessarily need to power taper. If you do, just a little.
4&5. Essentially yes but clearly a passive crossover would be best for less complexity (just a single set of stereo amps and less wiring). An active crossover would offer more flexibility if you like to tweak.

Now for your questions:

1. One enclosure is usually preferred but it usually partitioned with internal braces to strengthen the cabinet. You could thicken the cabinet walls as an alternative but braces are really superior in my mind to a cabinet that weighs twice as much.
2. Line arrays are more complex to design and build than other speakers so one would not go into them without either build a proven design or having the expertise to work through the details.
3. Selah Audio (and several other small custom builders) is a custom builder that builds speakers per requests from their customers. So you'll see far more variety in speaker designs and finishing options vs. a company that has a large factory assembly line operation to satisfy a specific product line for their dealers. Selah builds turnkey fully assembled systems and can supply parts kits for customers who wish to build their own enclosures. It is a lower overhead, lower cost, and more flexible way to do business versus high markup high end audio.

Jim