Hello Dave,
I understand both Windows and Linux can't access or have the drive mounted at the same time. Unfortunately the behaviour I am experiencing is the opposite of what you just described. That is when I have the vfat connected to and mounted on my windows machine I can run rmmod, losetup, etc. The windows drive is automatically disconnected and I can successfully mount the vfat on my Edison's file system and umount, modprobe, and reconnect my PC without restarting the Edison.
It is when I don't have my PC connected to the vfat partition that I run into problems. I cannot disable g_multi with the rmmod command and when I mount the drive (skipping rmmod) I can use the vfat space but cannot umount it and connect my PC without restarting my Edison.
David