Even for this you probably wouldn't want to use #2, because some grapheme clusters are composed to create ligatures and wouldn't be used outside of composing to create a ligature, so they shouldn't ever form a valid string by themselves. But by the #2 analysis this would read as "length 1".

Can you think of a way to do stricter while still not rejecting valid names?

I don’t have one.

(Except the airline model of “only A-Z” but it seems silly to stay there just because people could slip ligatures through the form)