adding a helper=crypt to fstab will make umount impossible:
1) umount calls umount.crypt with a -t parameter which make umount fails
but /run/cmtab seems updated anyway (the entry disappears)
2) umount.crypt will throw "No vfsmount found while searching... /run/cmtab"
Since the filesystem can't be remounted a second time ("File exists"), the filesystem is rendered absolutely un-umountable.
Resolution: umount.crypt must support the -t flag as any umount.<helper> must do.