A fucked up tank swap. It's can be a pita. They'll get better at it the more they do it. It happens. Statistically, it's going to happen.
Yes they share health. If you had a player who could do a bajillion DPS and one shot Akkelion, it would (since overkill is discounted), take Xhul down to 60-65% (I forget the number). DPS should be done to all three (with kill order being Akkelion, Omnus, Xhul). The rationale for focusing the adds though is (1) they are level 102 rather than boss level and (2) they will pass specific ability specific percentage thresholds that allow your raiders to do more damage (i.e. a destro warlock can havoc xhul +shadowburn akkellion)
You don't need to. This tells you nothing. Aside from bait team, they should be stacked EXCEPT when running out with fel surge anyway.
You need to move your range in more, so they are closer to the boss than the bait team. However, especially due to the patch, it is entirely probable that Omnus will spawn at a different, earlier point in Akkellion's ability cycle, meaning ability pacing (at least a certain point) will push Black Hole to be cast when the Fel Surge group is running back in, having dropped it off.
If that happens, or your solo soaker has feltouched, just have him swap with the next person on the list who can do it who doesn't hav efel touched. You can also see where the blackhole will spawn well before it starts sucking the raid in, so move 20 yards away then if it is a bad one.
But really, try to arrange for it to spawn near bait team. If it spawns behind the rest of the group, that should be an unfortunate accident and not a regular occurance.
Looking at your comp I would do:
(1) Rogue
(2) Spriest 1 or DK/MW or Warlock
(3) Spriest 2 or Warlock
(4) Whoever is the most competent and durable ranged DPSer you have. Else Warlock.
(5) Rogue
(6) SPriest 1 or DK /MW whoever did 2
(7) Spriest 2 or whoever did 3.
(8) Might not get this one anymore. But whoever.
4 is the only toughy in the fight really because Chains go out likely before the soaker can get back in. So whoever is out there needs to be survivable for that too. Warlocks are a good choice due to number of defensive CDs.
As for which ones warlock should use assuming Disc Priest shields are there VW+Command Demon, Unending Resolve, Sac Pac in various combinations all work, but I find that for simplicity's sake, Dark Bargain is the best choice to soak it. The Warlock will need to be watched a little when it starts ticking, but it's no big deal. If you don't want to do that, VW+Command Demon + Unending Resolve + Disc Priest shield will leave the warlock with about half health give or take.
Thing is, since you don't have 3 mages, Warlocks may be better with Soul Link than hunters for chains team. So maybe instead put him at bait team and have a hunter deter the black hole (I think that works?). You should also break the chains in a contolled, ordered fashion, not quickly.
As others have said, doing this fight without DKs is going to be obnoxious.