I think the problem with the clean code is that a forcible unmount
   results in a "clean" marking.  The clean marking should only be applied
   if the mount would have succeeded without the force flag.

No, that's wrong.

If you still have an inode in use that's unreferenced, unmounting with
MNT_FORCE should cause it to be garbage collected, and the file system
will be fine.  If you're not going to set the clean bit in this case,
then there's no point in using MNT_FORCE at all.  In my environment,
at least, that would completely defeat the point of the clean bits.