Hiro Mashima wanted it canon he gave us a back story of Acnologia, introduced the new characters like brandish and Invel, mentioned the girl from the movie in the manga... I think that is more than enough for it to be considered cannon.
He even said he would of added more things in the manga from the movie, but that he simply didn't have time.
not really an assumption since it's made pretty obvious honestly. That said if Hiro wanted to introduce the thing in anime and was pretty much the producer of the movie don't see why it shouldn't be considered canon.
That said the movie itself was a little dissapointing to me...
yeah, they actually dropped many "characteristic" FT stuff after like the battle of FT arc like: Gray accidentaly insulting Mirajane, than the 100 time pay back after the battle with hades which became a rather common Natsu phrase before he is about to kick someones ass and even the entire thing of Gray and Natsu pissing themselfs in front of Erza. Like they still do, but it aint the same.