Not plate tectonics, lava flow expansion. Yes, it was Red Mountain. Happens near the end of the third Era. That Imperial general lives for another 200 years after the eruption.

So first, Vvardenfell has definitely been an island for most of the TES games. The Red Year event, which yes involved Red Mountain, may have caused some cataclysmic changes to the landscape, but I honestly don't know. I think the most important thing to keep in mind, though, is that we can't expect the cartography to be 100% accurate between games because different games need to focus on different environments at different scales, none of which should really be considered "the most canon version" as all version of Tamriel are incredibly small microcosms of a purportedly canon world. Unless we all really think the the entire population of Tamriel can be supported by the like dozen tiny farms we find in the world. It's supposed to be evocative of things that exist, not a perfect scale model of the universe.