I don't see how Antigonids would be able to reunite Alexandrian empire - too many open fronts, always someone ready to stab in the back when occupied elsewhere. I'd rather see a some cold of cold war between Antigonids on one side and allied Seleucids nad Ptolemeids on the other, from time to time blowing hot.
It'd be interesting how Hellenic-Rome relations would develop in above scenario - assuming, of course, no major changes in Roman history.