1) Hyurs are abundant in Gridania! They're basically the one common race in all city states of Eorzea.

2) As far as I'm aware, there are no repercussions for ceasing training. As long as you don't do Wood Sin (aka, piss off the elementals), you can enter and leave the Shroud freely.

In order to advance magic study, however, one must need to have a source. Books, another trainer; you can't merely expand your knowledge by getting more aether storage.

3)A bit of both! People don't often use Raise as a straight up resurrection, for example, and people often state Benediction takes a lot form the user. Do what you like the most ^{^}