Mini-campaign clinic: fronts and intro moves

I have a precious school-free month coming up and plan to run exactly four sessions of Dungeon World with friends (all experienced roleplayers with varying Apocalypse Engine experience). I plan to run a mini-campaign with a satisfying conclusion in that short duration. I'm basically skipping Acts I and II and starting at the beginning of Act III. Here's how I think I'll do it:

1. Plan really good introductory moves

2. After the first session, develop one or two very urgent fronts

I'd like help with both of these things. Specifically, how do I find the sweet spot between enough detail to kick things off with momentum and enough vagueness to respect my players' contributions?

I get the sense that I'll want my intro moves to provide me the detail I want by having the players supply it, with me simply suggesting urgency and consequence without defining it.

Am I on the right track? Any advice how to accomplish this? What am I overlooking besides all this?

I had an idea. I think I'll pretend some grim portent (a big one) has already come to pass, and tie in the intro moves to that portent, and include "Mark XP" in each one to give them all an advancement headstart.

Potential examples:

• You manned the walls of the Bright City when they fell before the shimmering horde. Describe your foes and what you sacrificed to escape, and mark XP.
• Had you returned in time, the artifact you recovered from the tomb of High Bierbalang might have averted the massacre at the Bright City. Name the artifact and describe its appearance, and mark XP. Roll+INT. On a 10+, choose 3. On a 7-9, choose 2:
  • You didn't lose the artifact when the Bright City fell.
  • You know how to activate it.
  • You have what you need to activate it.
  • You know what it does.

Something along those lines. So far I've had someone call dibs on the Fighter, and another on the Paladin. We've got 2-3 more players who haven't stated a preference.

Thoughts?

EDIT: I had some thoughts. For four sessions, there's no reason not to do these for every session. I can come up with a new set after each session based on fiction established during play, pretty much right after I make or update my fronts. It'll be like the flashbacks in Lost. I can start with fairly vague ones and bring them into clearer focus with each iteration as the field is seeded with story bits. I think I'll keep them focused on Acts I and II, and then incorporate their fruits as the adventure culminates.

I'm thinking maybe the party has been sent to investigate and secure the fabled Deep Sanctuary as a place to house fleeing refugees, and we'll start play at the entrance. I'll make another intro move dealing with how this mythical ruin was recently located (I'm sure we'll have a bard or wizard for that one). The refugees are only a few days behind, so the task is urgent, of course. I think I have a good start now.

I guess writing it out on the forum did the trick, eh? Well begun is half done, indeed.