The libpython27.a is an actual 32-bit version, as confirmed by running objdump -t on it.  It reports the sections' file format as pe-i386 instead of pe-x86-64.  I am only using it for building for the 64-bit target, so I cannot confirm its viability for 32-bit builds.

To build 64-bit Windows binaries, you need a MinGW-w64 toolchain (which is a completely separate project from the original MinGW project).  The target triplet is x86_64-w64-mingw32.

I cannot speak to the stability of the MinGW-w64 library interface, but I would expect it to be stable.  To wit, I would be shocked if future changes required regeneration of third-party .a files.