I think that could happened. The south decided to be independent to stop attacks from near harad. Ilthilia left after feeling that gondor just left them to deal with Morgul valley. Dol amroth decided to stop being a vassal to a nation that doesn't help them fight umbar and I'm not sure if gondor have more land so that's it for now.
But it's a late Third Age alternate history. Gondor was fully united at the very end of the Second Age, so it doesn't really matter. No matter what happens differently in the Third Age, Gondor would still own all of it's land in the beginning of the age.
Also, Ithilien would never survive on it's own on the time period, and declaring independence would be suicide. Gondor does help Dol Amroth fight Umbar. Gondor invaded Umbar during Aragorn's lifetime, back when he was known as Thorongil.