I don't think it is useful to have a feature for setting a version
that is flagged sighted as unsighted. The point is: if the version was
flagged as sighted and shouldn't be, then it contains vandalism. This
vandalism should be removed and thus a new version should be created,
which automatically gets the sighted-flag, but this time for the
correct reason. The the last editor should then approach the editor
who set the flag wrong.
For other flags this is indeed more complicated. In the case of
reviewed, the possibility of storing all flags seems to be reasonable,
as well as the possibility of removing flags. If a flag is removed, it
should still be possible to see this in some way, for the reason James
mentioned (blame purposes).
As for the rest, I agree with James, flasg from different users
shouldn't be weighted.
Bye,
Philipp