Well bro, Please don't take any offense (As I don't intend any), It was stated that Curse is a TYPE OF MAGIC during tartaros arc. Also PureHito (Someone whom Mavis' acknowledged as having a great deal of intelligence and knowledge) said that Magic's source was negative. While I wont justify him or anything. I don't remember anyone (maybe, other than Natsu) growing "stronger" for their friends, they just got some willpower. Well, that said you could be correct as well, or maybe it can also be that curse and magic are linked together. Whatever It is, I believe that Natsu's END mode could very well be compatible with his dragon force.
And even if curse is incompatible with his dragon slaying magic, I believe his "container for magic and curse" will be very large in his Etherious form and then he could ditch his curse power and use magic instead (INSIDE THAT INSANELY LARGE CONTAINER OF HIS)
UPDATE: Well the feelings that made Natsu become stronger could also be negative feelings like (Oh shit I'll loose her forever) Sadness , Fear, Anger or (the HxH way) One's Resolve. This further supports my theory while also explaining Nakama power-ups
---------- Post added at 04:00 PM ---------- Previous post was at 03:53 PM ----------
I feel One's resolve is the best way to look at it.