We know that Merc doesn't have his full power. Him correcting her could just be to set the record straight that he's not completely useless. We don't know how much power he has left, true, but the fact that he was in a very low-power state before the lacrima, and is back to not having his full power after it was destroyed indicates that he went back to his new 'normal', since he shouldn't be having any power issues at all, otherwise.
If we assume that he has a large amount of his original power back, then why not just say so? Why not alleviate Karmeel's concerns instead of keeping his usefulness vague?
Regarding my example, that was to show that Merc can't dish out high-powered attacks when having a limited amount of his mp. For example, if he has 30% of his magical power, he can't use that to hit as hard as he normally would for a limited amount of time. If he doesn't have his full power, then any actual full power DG would trash him.
This doesn't really dispute what I said previously, though. We're currently debating whether or not Merc is a huge buff, so to me it doesn't seem like much of a coincidence. The Merc stuff is being portrayed as him being used in a fight, not to specifically contain Natsu's flames (which he should be incapable of doing based on their last fight). She could just try and hug him again or something, assuming it comes to that.
Lucy's ominous narration could be something related to Natsu, it could mean that either Aquarius or Merc ends up dying, or it could mean nothing at all like the Alverass one. I have a feeling it'll be the last one, but we'll have to see