Certain clarifications that need to be made: FT doesn't follow the one-timeline logic but the multiple-timelines logic. Carla simply "made contact" with herself in that one timeline(call it X) where X-Carla somehow was already capable of casting said spell.
As for the whole she was supposed to die thing, well, apparently she was in a hurry and misinterpreted her subsequent blackout as death OR in timeline-X there was no Doranbalt to save them and they truly died.
With that said, this is easily not classified as good writing. There are myriads of situations where Carla's power could have been put to use , to the point that one can say she has the potential of learning every magic. It's too convenient with no established rules as to it's limitations , unless you count some sort of abstract "oh I can't control it properly" as a limitation which works and doesn't work according to Hiro's will. One thing's for certain and that is that Carla's ability has been used consitently for fake mommentary build up.
Funny thing is how everyone loves this chapter cuz Wendy is involved. If it was Natsu everyone would shout in sync "king of asspulls". Seriously what's the matter with you folks and lolis???