It was similar to the already existing MSYS2 and "Cygwin", except microsoft added a program that implemented Linux's syscalls, to allow the execution of GNU/Linux binaries without having to recompile them first.
3. GNU/Linux was never that unstable and buggy - GNU was on track to achieving the lowest error rate and least amount of bugs for any OS before Linux even existed.
There was a lot of proprietary sabotage that was a pain to deal with in the past, but that has been mostly worked around - too bad microsoft and co do their best to add more.