1. no, that should be ok, just paint it so u cant see the line.
2. should be fine with 1.5", its pretty much preference there. I would put tops on 'em.
3. not to sure, but i would buy some rosewood or ebony and just make fretboards to put on top.
4. not sure about that one. sounds like the plan you have is pretty good tho. i think if you wanted 2 more necks you would be best off buying a little extra wood seeing as how you got such a deal on this.
-RAF