-1.0 will have a slightly different architecture so it would be a good thing to use a clean RTW 1.5. In fact ever since EB used the mod folder architecture that has been the case (requiring a clean 1.5 build).

-No unofficial mods have been fully incorporated; and unofficial formation mods have been left out completely.

-It sure will. In exactly the same way as it is now.

-Not as unofficial modders; but you are right: some people who have worked on their own unofficial mods also have been/are beta testing the build.