I've tried and yes:
error: failed to commit transaction (conflicting files)
libfm-git: /usr/include/libfm exists in filesystem
Errors occurred, no packages were upgraded.
But then first I've removed pcmanfm-qt (with libfm), then installed libfm-git and then pcmanfm-qt without any issue.
Awwight.
I had to go one step further:
I removed pcmanfm-qt, libfmfm-extra, lxqtrunner, lxqtpanel, menu-cache
installed libfm-git
then I restored missing pieces, removed gtk2, reboot. Everything works fine and I'm gtk-free

Thanks!