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Good luck. Cygwin has explored this space thoroughly. If there were a better option that preserved compatibility, the project would use it. Keep in mind that Cygwin already uses the native API extensively. Read the code.

As for fork: believe me, I'd be happy if, as as side effect of this work, more applications used posix_spawn. But Cygwin can also support an efficient posix_spawn implementation: https://github.com/dcolascione/cygspawn

A lot of the complexity actually comes from mapping POSIX filesystem semantics to Unix ones. stat(2) in Cygwin is ungodly expensive for this reason. Your layer won't be able to avoid this work without providing fewer features.