I would say up front if you're not interested in figuring out any problems to get the project working for you, you're probably not interested in using it. If you are willing to spend time diagnosing scripts, build settings and fixing the frameworks, welcome aboard.
1) If you have existing binaries of the toolchain (the executables built with install.sh) using a current version of Xcode works (I use Xcode 6.3 on Yosemite), maybe you'll need to tweak some build settings. Building the toolchain on current versions of OS X is problematic and I am not aware of a simple solution. I guess I should probably make these binaries available for download to make it easier.
2) Sure, why not, it's fun.
3) I can't really say what Apple is going to do here, Xcode 7 could make using external compilers easier or harder, I'm not going to guess. Worst case is writing/using an alternative build system. There is no inherent dependency on Xcode other than management of the project, it could all be done with makefiles and an editor if needed.
It is possible to use clang but that requires some manual work until the scripts are sorted out for it, they're mostly there but haven't been tested much. Some people are using that so if they have any pointers that'd help.