From the list of answered questions (stickied in this forum):

Can a hero target his own square?
No. (Except via Dark Charm.)

From the JitD base rules:

Melee Attacks: Melee attacks can only be declared
against adjacent spaces.

My conclusion: When Dark Charmed, the OL cannot force a hero to use a melee attack on themselves. Ranged attacks and Magic attacks can be so forced.

Thoughts?

I think that the Descent designers could save a lot of grief if they just declared that any space is considered adjacent to itself. There have been a bunch of cases where things that affect adjacent spaces were errata'd to say "and we meant the same space, too," and in the few cases where there has been an official response that said something other than that, I think that extending the effect to include the origin space would have been a better idea.

In this particular case, things are rather muddled even apart from that, but I think RAW actually supports Dark Charmed heroes making melee attacks against themselves.

The FAQ makes it clear that heroes are not normally allowed to attack themselves. Attacks target a space, not a figure, so this is most reasonably read as meaning that no attack is normally allowed to target its origin space.

Dark Charm says that you have to follow normal attack rules, but it also explicitly says that heroes can attack themselves, and the FAQ makes it clear that this is supposed to supercede the normal restriction against targeting your own space. So I think it's reasonable to suppose that this trumps the normal restriction for all attack types, since that restriction exists for all attack types.

I would say your conclusion is incorrect. Melee attacks can absolutely be forced using Dark Charm. DC is an exception to the rule you quoted from JiTD. You even quoted the supporting statement from the Gathered List of Answered Questions (a collection of official answers from the game designers).

Plus, the quotation for ranged attacks doesn't really work in your favor, since by that limited definition Reach attacks wouldn't be melee attacks since you can target a nonadjacent monster with a Reach attack. That line is in the rules to distinguish it from ranged/magic attack which can affected non-adjacent targets.

Games like Magic: the Gathering and Cosmic Encounters teach us that the text on the cards overrides the rules in the book. The book rules set the general guidelines on how to play the game and the special powers on the cards (in the case Dark Charm) give us the chrome.