But like I said, it was shown more than once that using an external source doesn't allow them to have it permanently since Wendy had to train to obtain it and there's Natsu who just used his emotions (which seems to be his main source power up

) to first use his back in Tartarus. Correct that he did changed the property of his magic to match that of his Curse but that is pretty much how he got what was assumed to be DF since his his magic done altered to be that of a Curse power along with it also being thanks to it containing iron. Pretty similar to Wendy who ate her element which is air that contained Ethernano that powered her up. But since it was assumed that's DF he used, then it is an external source but only for him though.
How's that the same when Invel has shown to have take injuries by Gray's Zeroth's Long Sword twice to change it into a blizzard, and Ice Lock, plus some attacks that he used before he got defeated by the POF of Zeroth's Destruction Fist to use his trump card?