Well like I said, the mode allows him to regulate it better. We've seen that he can damage his entire arm if he uses too much of that power
Now the damage likely won't be as severe as that, but it could still occur.
Also like I said, he was amping himself with that ancient magic that Erigor was talking about. It's not as if Natsu can generate that kind of heat whenever he wants to; he has to get emotional enough to subconsciously activate that magic. As for dragon Force, like you said, he's physically tougher in that form, so he can pour in more power without having to worry so much about his body being damaged(his body
might also be able to naturally handle the heat better, due to it being more like a fire dragon's, but I don't have anything from the manga to prove this to be the case).
Well DF Natsu could easily burn through Zeref's weird tentacle attack,while base Natsu was struggling greatly to overcome it, so that could be proof of DF also amping his flames' temp.
But if Natsu can use the flames in his base without the mode, and if his flames are as strong in base as they are with the mode, then entering the mode did nothing other than deplete his magic faster. Natsu may be dumb, but I don't think he's
that dumb.
Well I don't know what to tell ya man. Erigor outright says in that scan that Natsu was using Ancient magic. Also, I agree that it's not a mode (or at least, not like any of his other modes);. I see it as sorta like Kaioken. He's briefly augmenting his power in his base form to a great degree and can probably stack this magic on top of his actual modes if he tried (although I don't really have evidence from the manga regarding this, too)