Is Akavir an archipelago, or is it a just a land mass? Maybe a combination of both? I don't remeber where, but I read that Akavir is a large collection of islands.

I'd guess a combination of both. Terms like "Thousand Monkey Isles" and that "innumerable reefs, sandbars, islands, etc. that infested the coastal waters north and south from Septimia" point out that there are a dang lot of islands there; however that the Anuad places it as one of the four major lands that Nirn settled into, its probably also a decent sized place...

Thras is a large collection of islands (actually, coral reefs....but a few are as big as islands), but it's not part of Akavir.

I wouldn't say Akavir is an archipelago; more like a large continent with a collection of islands in it. But not all of it is islands, and we don't even really have an accurate idea of how big Akavir really is.

Akavir is certainly based on Japan, right? Weopons, armor, architectecture, and all.

Yes, but I'd say throwing down Japan over there wouldn't quite suffice...

I'd say yes and no, but there is extremely limited information to make a decent enough judgement. All we know is that the Tsaesci has Asian, and a minor bit of Roman (I find the armor resembles rome more) influence in their armor, weapons, and building structures. As for the other denizens of Akivar, we really do not know.