What do we really know of the Left-Handed Elves? Aside from them building the Orchalc Tower and being the enemies of the Yokudans? Were they Padomaic, a reflection of the Man versus Mer / Auri-El versus Shor dichotomy of Tamriel, their name being more figurative, meaning "wrong" to the sword-wielding Anuic Yokudans?

This is the first I've heard of them, but I'm not sure how to take the fact that left-handed elves have a name that sounds an awful lot like "sinister".

Yes. Sinistral means left-handed or facing the left side and has historically been used as the opposite of "right" because it is different. I hypothesize that the Yokudans - a race that wields swords and has swordsmanship as an integral part of their culture, would use the term "left-handed" to mean something wrong or against them, those that are in the right. For an Anuic race that basically worships Raptuga, Tall Papa - the Eagle, the Auri-El of the Yokudans, a race that is Padomaic and follows Shor, the Void between the Stars, would probably be the definition of "wrong".

I also have this crackpot theory that the Lefthanders were behind the corruption of the Hiradirge, perhaps even sparking the civil war in Yokuda and through that, taking their revenge, or perhaps serving their master. We might be seeing them again in a Hammerfell game.