Immidiately I’m thinking: “This is bloody* good!”
Then I got to think, and the arguments that it’s really not less simple than a Fight!** is completely valid (even though I understood the dividing of pools-bit). I think your problem can be solved with a simple Bloody Versus, but with careful intents and stakes***, and those not helping could possible accomplish other things - or get advantage dice to their test for fleeing/hiding/whatever.
Example:
So, we're about to deliver a ring to Mount Doom. The Nine of us, enter the mines of Moria, and we fail our Read-test and get ambushed by goblins. Our failed test for (untrained) Goblin-wise (thanks a lot, Pippin!) means that they have a Cave Troll with them.
Boromir and Aragorn takes charge and tries blocking the doorway. Boromir rolls [i]Power[/i], Aragorn helps. Their intent is to keep the goblins out (for as long as possible), at least until they can mount a defence. They succeed, and we don't get disadvantge-Ob. for the Bloody Vs. leads with the goblins. Aragorn test s his Sword Vs the goblins Spear w/help and Boromir, Pippin and Munti help with Sword and Knives. Their intent is to win/hold the goblins at bay and they tie with the goblins who's intent is to spear everyone in the Fellowship. Even if Aragorn and his helpers lose, the goblins will not get everyone in one test - most importantly the Ringbearer (Frodo) is safe (or as safe as you can be fleeing from goblins in Moria). Amazingly, due to excellent rolling and great Sword skills, Aragorn tie the goblins. The GM considers them the defenders, so the tie go to them. They'll be continously hounded until the Bridge of Kazad-Dùm the GM says, and everyone is happy.
Note, that if Aragorn &Co would get hurt, different MWs and the application of armour-tests could ensure that not everyone suffered the same kind of wound or a wound at all!
While Aragorn and Boromir with help deal with the goblins, Legolas says he’ll take on the Cave Troll, to stop it smashing through the defence. He wants Gandalf to help, but Gandalf is either turtling or testing his Bridges of Moria-wise so he declines. Frodo volunteers though, and they enter a Versus with the Cave Troll. “If you fail this,” says the GM. “The Cave Troll will smash through the door giving the goblins an advantage-die against the defenders and the Ringbearer will be skewered for a Mortal Wound!” (Legolas has a Belief about protecting the Ringbearer, so he spends three persona and cries: “NOOOOOOOOOOOOOOOOOOO!” … when the traitors appear. Frodos instinct of keeping his elven chain hidden gets tested, but he rolls all the dice and successfully survive beeing skewered by a cave Troll’s spear.
I do agree that the Bloody Verus is a bit difficult to get. Last time I used it I was defending against three orks and just spent all my Beginner’s Luck-dice in the defence-pool, as it would seem that I got my intent (Survive the clubbin’!) if that succeeded. As a GM I’ve been more inclined to make a simple Vs roll, and maybe grant a test to avoid beeing hurt.
*) Excuse my lack of english explicits.
** Yes, I still like to have the exclamation mark.
*** “failure clauses” we’re calling them.
What I’m also concerned with, is that as this idea is much more complicated than a simple Bloody Versus, and not neccesarily much less than a Fight! - you really get a lot of rolls, but not the energy and intenseness of the Fight! - and not the nice oppourtunities for getting sweet tests and using Artha.