The error regarding the wrong absolute path names is related to the prebuilt modules

They have been built with system-file-name-convention set to posix. In fact on mingw-w64 this value is set to windows. I forced

recompilation of related modules by

For i686 the compilation process succeeded, for x86_64 recompilation of modules depending on boot-9 failed.

I recommend not to compile the posix/windows setting of  system-file-name-convention into the prebuilt modules in boot-9.scm:

Instead it could be evaluated during runtime. Otherwise formally different prebuilts are required, e.g.: