I've also read that Seleucia was the capital before 240 BC. I believe that the reason for this placement of the capital is for the AI to focus ont the west and the mediteranean more than on the east ( as in RL)

The seleucids were called syrians probably becouse their powerbase was in Syria, and that never changed throught history.

I also never heard that the romans actually concluded an alliance with the ptolemaioi. All i've ever read about was that the Ptolemys sent emisserys. I don't believe they concluded the alliance since the Ptolemaic kingdom helped carthage during the punic wars (at least one of them with money)

For the Arche thing. I asked that a long long time ago. IIRC the Arche in this context means something like the Mighty kingdom of Seleucia


hope anything helps