Are you saying Brian can't expel fire magic from his mouth when he can even create mouths on his hands? There's no need to argue about something that's obvious: it's a fire attack.
Yes, they had Dragon Slayer properties before Natsu received Ignia's amplifiers, and all of them were quickly rendered useless by Mercphobia. They were only able to do anything after Natsu dealt a lot of damage to Mercphobia. Natsu was responsible for Mercphobia's defeat; just look at the chapter. Gray, Erza, and Wendy contributed minimally.
Furthermore, this rhetoric doesn't contribute to your argument; quite the opposite. If Natsu, amplified by Ignia's power, needs Gray, Erza, and Wendy to defeat Mercphobia at less than half his power, due to Faris and Dogramad's nerfs, why would Lucy and Brandish be able to defeat the 100% version of Mercphobia with less damage than Natsu dealt to him?