Who said Shirin was a throwaway character? She had one purpose in Azadistan. Making Marina more than a simple figurehead leader. Muslim nations do not exactly have great reputation for their women's rights (except Pakistan, one example being the late Benazir Bhutto) especially in the areas where Azadistan is supposed to be. Marina started the series coming off as more of a concession given all the strife the region had.

However, Shirin proved time and again early in the series that she was a very shrewd and capable woman in politics, even if she lacked the position to truly use her intellect and insight.

I took her leaving Marina and changed appearance proves she has a significant chance of being a major player in the second half of the series. Why would the creators bother with the appearance. This isn't Kiddy Grade, where people change appearance (entire bodies) like they change clothes.

I viewed that scene as Shirin deciding her work in Azadistan was done, and now, particularly in a new worldwide government, a person of her caliber can now do quite a great deal. It's not just three large countries with everyone else trapped between anymore.