I'm sure I'm not the only one that's speculated about that? I mean, the similarity of the names are pretty obvious... Now, of course, little is mentioned in lore about Tosh Raka, but still, what is there suggests he was a chieftain/king of sorts, that somehow turned himself into a dragon - now that sounds familiar, doesn't it?

So, is there any educated guesses on Tosh Raka actually being Akatosh, and the consequent impacts that may have on lore?

Idea came to my mind as well not so long ago, including the idea that he may have not turned into a dragon literally but that he also became a dragonborn and myths just escalated to that point. But I don't think I saw a single thing to prove me right so who knows.

Mantling, maybe? Could one mantle Akatosh in the way they could Lorkhan?

Yes. It is likely that Tosh Raka pulled something similar to Dracochrysalis like Auriel did.

Different aspect.