From what I heard, the last time they "revamped" they were pushing toward the Fury more as a DPS class than a healer hybrid. I don't doubt the fury could do her fair share of healing - my main is a fury - but with mercs there's no need. Just grab a healer merc and let the 3rd slot DPS.

Mercs are allowed in instances, but they take up a party slot. So 3 regular toons with 3 mercs = a full party. If you want to invite other players, you'll need to dismiss a merc.

As far as Dirge vs Fury, Dirge would probably do more for you. Warlock already has ports and invis, so you don't need Druid Rings or Untamed Shroud. The Dirge gives you tons of buffs, plus movement speed and better DPS from my understanding {I haven't touched my dirge in years}.