> That release is intended for building extension modules. Is it possible to configure it for building CPython

It's intended for extension modules, but should be usable for building CPython itself if someone wants to take the time to update the Python 2.7 build system to use it. (Also, there's nothing wrong with the 64-bit toolset in this installer - that was the Windows SDK version of the compilers.)

The main reason you can't use it is because our build system is MSBuild based and the compiler package does not register with MSBuild (it doesn't provide a Platform Toolset). It would be possible to write one that would work, though I don't know exactly how to do it - I believe ICC and clang both provide the required files though, so it may be relatively easy to add the toolset files into our source repo and make it work.

There are likely some other reasons that the build will fail, but those can likely be worked around within the source repository.