A quick update:
I was after all inspired by the wealth of input on this thread to do some extra effort and make a (very rough) GPlates model of the plate movements. So far I've roughly drawn the "main pieces" in their present state. I also changed some of the plates to use their "native names" (when I had such figured out). Anyway, this is what the draft looks like at this point:
Next up, I'll try to move the plates around to show their arrangement in the "supercontinent phase". I think I'll decide where to place the (former) Xeteyaric plate as part of that process.
@Pixie: I agree that the Urgaleon-Eocidar border should probably be moved closer to its previous arrangement. I haven't decided what to do with the islands west of Eocidar at this point. One possibility would be to arrange them as a minor continent that is largely submerged, similar to Zealandia on Earth. In that case, they would be a "drifting continent" that wasn't part of the most recent supercontinent but rather had broken off at some earlier point. In this case it would make sense to make the micro-plate they rest upon a part of the East Agalhaic plate. Although I also like your idea about a super-volcano.
@Durakken: The Agalhaic plates closely mirror the Pacific and Nazca plates on Earth. In other words, they're part of a single oceanic plate that has split in two because it is subducted on both sides by continental plates. Speaking of subduction, my understanding is that subduction is actually the main force driving plate movement (and ridges and rifts occur as a result, rather than being the driving force). If you look at Earth, the East Pacific Rise is expanding about 15.1 cm/y at its most expansive point. By comparison, the Atlantic ridge (where bordering plates are subducting rather than being subducted) expands a measly 3.5 cm/y at its most expansive point.
Edit:
@Pixie:I just realised you were talking about the island (almost a micro-continent) east of Eocidar, rather than the western islands. The eastern island (called Dealos) is actually rather important in the grand scheme of things, as it's essentially the "Atlantis" of this world and plays a major role as the land between the major continents. So, I'm definitely keeping that one. As for the geographic explanation, I suppose it's similar to Madagascar (broken off India), a piece of the Nimro-Druegian plate that broke off to join the Eocidarian plate. The sea between Dealos and the Eocidarian mainland can be explained as having been caused by a failed rift. Although there is also a hot spot there that has created the long island chain east of Dealos, and probably the southern portion of the main island as well.
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