Hi,
I have been unmounting already several months using the SendSignal executable and killing the mount window. No sign of lateral damages by now.
On the other hand, have you had a look to my reference this one
It is providing a way to unmount WinFSP, perhaps a way to go?
They claim:
The proper way to unmount a file system that uses WinFsp-FUSE is by sending it a Ctrl-C. On Windows this is done by calling GenerateConsoleCtrlEvent