The reason I think the Bell has the edge over other tap to draw effects is:
(1) Cheaper than the next best thing, Jademeadye (sp?) Tome;
(2) Can deck your opponent if you've been capped or something; and
(3) Is an artifact, so easier to fetch for blue without wasting space in the deck.
It's actually MoM that is the limiting factor, like you said.
Regarding disenchant effects on Bell/MoM, your first scenario doesn't work. You pitch to untap, they respond w/ Naturalize, and you try to tap Bell in response. This doesn't work because the untap effect still has not resolved. You will always have to use the second solution.
Are you sure? Stack ends up like this:
(1) Tap Bell to draw a card (bell becomes tapped as a cost)
(2) Opp casts Disenchant
(3) Discard to MoM to untap bell
Allow (3) to resolve, then, before allowing (2) to resolve, activate a new (3):
(3) Tap Bell to draw a card
And you're off to the races.
This is what makes this combo stronger than, say, solidarity, since you are not relying on one specific spell for the untap effect. As you say, though, probably not yet competitive.
EDIT: Oooooh nevermind I see what you mean. And the opponent gets to chose which effect to respond to, right? Still, that doesn't change much, since you will just activate MoM again in response if you can, and you're still good. K.Grip is the killa.