The answer to your question Sam2 & Israeli Chareidi is, that when a Tzibbur commences early Shabbos [if only synagogue in that locale] you are dragged after them, so it is full fledged Shabbos, whereas if a Tzibbur prolongs Shabbos after 3 stars are out, no-one is dragged after them, hence this time cannot qualify for Seudo Shlishis.
As for your question the Goq, absolutely not, otherwise we could solve some of our schduling issues on Shabbos Erev Pesach, and no-one came up with this idea, because Seudo Shenis has to be consumed by daytime.
As to your concern crazybrit, go to an early Hashkomo or/& Vasikin Minjan, have Kiddush & Seudo Shenis by 8 or 9AM, and by lunchtime & Chatzos/Mincho Gedolo, consume Seudo Shlishis with hearty apetite. & for another suggestion, keep a box of Matzos in your Shoule’s social hall, so next time you attend a Kiddush there, have a piece of Matzo transforming the Kiddush into Seudo Shenis, and lunch at home will be your Seudo Shelishis with a hearty apetite.