The project's pre-pre-alpha will be out very soon (in a week or less) and will already give a nice taste of its design and speed. The primary focus thus far has been major challenges (toolchain, process creation and initialization, signals, glue layer between posix system call layer and libc, etc.). This means that while you will be able to test functions such as fork(), execve(), mmap(), or fopen() with a utf-8 path…
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.