So you agree that Ignia can burn Natsu? His flames being special which we have no idea how special, means nothing. Since Natsu was still able to eat them, thus not making them the same as Zancrow's. Never was it stated that Invel's ice was special aaginst slayer resistance only that he can freeze ice which again just means he's stronger/colder then Gray's resistance. For Hakune, we literally see Hakune say Gray's DS halved the effect, you can't half immunity unless it's resistance. You posted a scan of Acno being motion sick not anything stating that motion sickness leads to resistance lowered, again even more proof that it ain't immunity since you can't lower immunity, but you can resistance.
Wrong as we see Natsu grows stronger the more magic he's able to burn, and he was built to kill Zeref, so using headcanon like you, nothing says his time burning fire isn't hax.
B.Zeref scaling to Phoenix is iffy, but that's besides the point as that will still make Natsu's absolute heat an outlier. Zeref with Neo wouldn't even scale to anyone in the first place. So again Natsu having that level of power by himself would be an outlier unless it's hax, which makes more sense.