If the drake being alive did not affect healer mana, we would simply release them and not bother killing them at all. You seem to be contending that at their worst, the dragons merely need to die before Halfus hits 50% and begins really driving us crazy with stuns. If the dragons were this unimportant, then everyone would do what you are advocating warlocks should do: essentially let the dragon die to splash damage. I am just not seeing how this is possible. Killing the dragons is the hump to get over. Especially when you have something like Malevolent Strikes that require your tanks to taunt back and forth, having a second dragon to deal with is extremely trying for tanks and healers both. And, while healer mana can be used at any point during the fight and getting Halfus to die quicker is in everyone's best interest, mana does not deplete in a linear fashion. The fact of the matter is that more damage is going out when there are more drakes alive. They will be using more mana per second at that point than at any other time, and since they can only cast so many heals per second, you want to reduce the time spent in a phase where RNG will wipe you. Once the dragons are dead, mana recovery is much easier and there is a good deal less raid damage.

And finally, I will concede that if you are having that much trouble dying to Engulfing Magic, don't use Glyph of Soul Swap. I think if you can't use the glyph, you shouldn't bother playing Affliction because Destruction gives yields higher single target damage anyway and is much easier to maintain during movement heavy fights. I will say however, that I am shocked you have so many problems with Engulfing Magic. In early weeks, our Resto Druid was often slow to move and did a good deal of damage to the raid before he got out of range, but he never wiped us, even in Tree Form. We eventually adjusted our strategy, which made the entire phase much less prone to spike damage -- provided everyone was paying attention to the meteor. I think what I am saying is that removing the Glyph of Soul Swap is really the least ideal way to deal with the mechanic, and you lose a lot more than you gain.