That could be just magic power, and FH has Space Time properties, not a regular magic powered attack, Going all out for Zeref with FH means his attack would not lose any fire as it would just be replenished endlessly. Which is warping space time with Natsu‘s flame, and that is beyond his control to not be burned himself by it and his soul, it actually does, it is the lack of the ability to provide conscious limitation of impulses and behavior as a result of overwhelming emotion, just like Natsu not putting a limit on his fire, which has been the case as Prolyusica stated had he developed a Magic tumor. Iced Shell is a cost, not a exceeding limit, not the same, Natsu could not control his powers in the sense of controlling the heat of his flames. Not really as Zeref was going to be fine in a bit as he said he underestimated him so all of his magic was not burned away, just FH, Natsu has the combined power of Seven DS, Acno was motion sick, paralyzed and lost his magic immunity. Zeref scales to Phoenix, simple as that, it doesn’t matter what he shows or not, appearance does not matter, it would be much worse with space time warping had they gone any further with heat at that temperature which would create a black hole. Well considering that he was still a dragon after he was defeated makes it clear that Natsu did not have anything to do with the Merc losing his power would still likely have something to do with Whiteout since he still has no magic power and considering the Girl with Glasses and Merc talked the results afterward, but now that I think about it, if not, It could be because of his flames that are from Ignia which is stated to burn anything, where as with Zeref it was right away at the point of his defeat that FH was burned away, since Savage flames is basically reality warping.