How exactly did Akivir come to be in terms of being an actual continent? Was it created the same time as Tamriel or what?

probably, or definitely

Some like to think that Tamriel used to be the heart of a "pangea" supercontinent

Ah, I always thought of it as a shadow of modern Tamriel. An oh well.

Well, Tamriel was far bigger in the Dawn before the war of the Elhnofey.

Perhaps it's similar to Earth. It all started off as one big land mass and over time parts broke off.