If that were the case, then why would he specifically have the characters point out that Mercphobia's still nerfed? He had the perfect out with this spell bringing them back to their original power, but he didn't use it. Merc has limited power, and we saw during his arc that he can get wrecked by the Diablos Trio while in this state.
Thing is, we don't know if Natsu's gonna lose control again and if he does, there's no need for Lucy to have DG-tier power to calm him down. She managed to do so without it last time + Dragon Merc's water couldn't put out Natsu's fire when he had a little bit of Ignia's power, and this new power up is hyped to bring him on par with full power Ignia.
Regarding Aldoron, both Natsu and Erza's there. Odds are that one of them (probably Erza) will end up dealing with him.
I could of course be proven wrong due to Mashima pulling a Mashima again, but based on what was presented in this and past chapters, Merc has no business being relevant against either another Dragon God, or 'Awakened' Natsu
I think you're exaggerating the "limited power" part. We don't know how much power Mercphobia has; we only understand that he has the power to fight temporarily. Karameel is not the best indicator in this discussion, as she believed that Merc had no power before the lacrima appeared, and Merc also corrected her.
And your example isn't a good one, as Mercphobia didn't have much control over his powers at the start of his arc. Faris was the one who had control over Mercphobia's magical power, being able to increase or reduce it.
Regarding Natsu, I disagree. Lucy mentioned that something horrible was going to happen a few chapters ago, and when she calmed Natsu down in the Mercphobia arc, she got burned. If Natsu loses control next time, the situation will be much more drastic, as it looks like the power boost he will receive will be massively stronger.
And isn't it a coincidence that Lucy gets a huge buff (Mercphobia) in the same chapter where Hiro is foreshadowing the super special flames that Natsu will consume?
Remember that it was in the Mercphobia arc that Natsu went crazy. The parallels are all there. I'm not saying that Lucy will beat Natsu, but it seems that Mashima is building up to a moment where Lucy will have to calm Natsu down through powerful water magic to contain his flames until he regains consciousness and masters his new powers.