I'm actually kind of surprised that they didn't change Cover to "Rush to target's party member side and take all physical damage intended for target for the next five seconds," to make it more distinct from Intervention and give it some classic FF flair. They might as well just give everyone a gap closer at this point, due to the popularity of the mechanic.

The issue with Oath gauge is that because both abilities that use it are defensive, losing gauge is kind of meaningless outside of ensuring that you have enough to Sheltron the next big hit. That's part of the reason why you can't find anything useful to dump gauge on prior to switching. They really should have just moved all of PLD's resource costs to gauge instead of MP, including Clemency (Shield Oath only) and Holy Spirit (Sword Oath only). That way you'll always have a way to burn off excess gauge, but losing gauge also becomes more meaningful. You could also have Divine Veil and Passage of Arms drain gauge while active. They could balance this out by making RA generate gauge, which would help out with the slower gauge generation in Shield Oath.

As a side point, I think that Hallowed's invincibility effect should only negate damage that would drop you below 1 HP, like the other two tanks, and that blocks should occur in Shield Oath only.