You'd be surprised how easy it is to slip a Vets' mind.
EA doesn't actually have many indie scout provinces, I've played in EA games enough to run into cases where players not getting a single scout prov.
I find myself building scouts from forts now and then and don't always max out mage production, especially entering mid-late game with periods of peace, no gold income growth...etc
Though really, there's no need to rationalize attacking Aby, agreements are non-binding, the specific circumstances grey enough, and the prize large enough, it certainly makes strategic sense to go for it in your shoes. So speaking from an outsider perspective I applaud, plenty of vets would jump at the chance in this circumstance, it's only a matter of time for Aby anyway
I was wondering when you or Arco would make a move.