Uhh chances are his capital is not Seleukeia but Antiocheia instead. Doesn't harm the Eastern bit too much, most of the troublesome provinces are at the max 80% penalty limit regardless, but sure does help when expanding to the West. Believe you me: upon capture, Halikarnassos can be an absolute pain to control with an in-experienced governor when the capital is in Seleukeia, but it sure is a heck of a lot easier when he gets orders from Antiocheia instead.

Gerrha tends to be troublesome with or without the capital seat in Seleukeia. And as you say: it can be captured any time you will, making it probably a low priority in what looks like a 3 front war.