I seriously doubt Zeref is going to outfight Acnologia without magical attacks. Sphere and Law aren't really the same thing. Law is closer to glitter than it is to Sphere.True, but Zeref blasting him continuously would be pretty dumb, a quick punch to the gut or something would be the strategy, maybe a blast to the back of the head. Fairy Law would also probably work, given Acno couldn't eat Fairy Sphere. Attacking with Curse might also be a good idea, though I don't know how well it mixes with magic. All that aside, I dont see why Zeref wouldnt get anywhere hitting Acno with his fists.
I call that version of Base Natsu, Savage Natsu, seeing as he's leagues above any incarnation of Natsu bar 7DS Natsu. So Acno or Aldaron being able to trash base Natsu doesn't really mean anything to me, because FH Zeref didn't fight that Base Natsu. His portrayal as Acno's potential equal still stands relatively strong, certainly enough so that he would be able to defeat Acno's weaker form.
Burning Zeref's magic isn't good enough, Natsu needs to go the full 9 yards or else Zeref will just be the same as before, infinite and immortal. You can't have less power than everything Savage Natsu had and beat Zeref, you can't take him in a battle of attrition, he's infinite.
As for Ignia Natsu's hands, alternatively, it could simply be that no attack IgniaNatsu landed on Merc had "burning hands" power behind it, because his hands weren't burning at the time. Natsu was clearly amping up when he went "i'll burn everything" and his scales went away the same time his flames did, so protection wouldn't really apply, because scaleless Natsu didn't touch the flames.
Sure, that would be impressive... unless Zeref is stronger than that dragon god(which by all accounts he should be), in which case overpowering Zeref is the better feat. Physical strength comes from magic power, you know that.
That was just base Natsu. Again, "savage flames" involves FDK Natsu with flames of emotion. This was base Natsu, and he had yet to give the speech of turning his guild's power into power.
Zeref's magic at that point was Fairy Heart. He noted that time itself was burning too. The flames were hot enough to stop Zeref from reversing time and just undoing all the damage Natsu dealt him. You don't have to kill Zeref to beat him. Natsu didn't. You just have to stop Fairy Heart from reversing time and then injure him enough so that he can't move.