Is it OK to made/ hear Kiddush before Maariv, begin the seuda, then leave to daven Maariv and then come back to finish the seuda? I understand that the Gra was opposed to this, but in fact the Gra was opposed to davening Maariv before the zman (on any day of the year), and the minhag of many people is not like the Gra.
If one has a choice b/w davening early and then making Kiddush, versus making Kiddush early and davening at nightfall (which is really the ideal time for Maariv), which is better?
One can make kiddush and daven maariv later although according to Kabbalah one should daven maariv before kiddush. You are correct in what you wrote about the Gra. Here in Yerushalayim many Rabbonim were against an early Shabbos.
O:CH 271: Mishnah Berurah 11, Kaf Hachaim 272:3, O:CH 235 Biur Halacha ד”ה ואםת, Maseh Rav 117 and Peulas Sochir there.