So the truth is out.
Fairy Heart is infinite magic.
But I have to apologise to Mashima-sensei for getting way too agitated over the plot hole of the 7-year timeskip. Re-reading the manga, I realised that Zeref told Makarov that he only recently decided he wanted Fairy Heart. Convenient way of circumventing the issue, but it’s a shit load better than leaving it as a massive plot hole.
Still not convinced by the necessity of Alvarez to counter Acnologia. Still not convinced by Face or Etherion as weapons effective against Alvarez (which Makarov pointed out, and Zeref agreed to a certain extent). Still not convinced that Ivan didn’t take the opportunity to enter the foreclosed Fairy Tail building during the timeskip to try and find Fairy Heart again.
Zeref is still a pretty lacklustre villain, and the series is about to embark on its usual predictable fights. I’m basically just waiting for all this to end with Natsu beating Zeref but probably not killing him. Maybe Mavis can kiss him and end his life like he put her in stasis the last time. Meh.