Early Japanese states and the first emperor

I've been looking into early Japanese history lately, and it is beyond confusing. When I say early history, I mean all the way back when it was called "Wa".

So, traditionally, Japan was founded by Emperor Jimmu on February 11, 660 BC. However, this is probably just legendary. From what modern historians can understand, Japan emerged as a unified state in the centuries following the BC/AD divide, and before then was made of about 30 smaller states, of which the only two I can find information about are Yamatai, and the precursor to moden Japan, Yamato, with Chinese records of these existing around the mid-3rd century.

So, if Japan wasn't unified by then, it means that the first 14 canonical Japanese emperors are most likely legendary, or that some of the later ones were just kings of Yamato rather than the whole country. Emperor Ojin, who traditionally ruled from 270-310 is the first emperor of the Kofun period, making him a potential "first emperor", but the first we have contemporary history for, and therefore the first one that can be verified, is Kinmei (r. 535-571)

So, what I'm getting at here is three questions:

1) What were the other 28 kingdoms?

2) Who was the actual first emperor of Japan?

3) When was Japan unified under the Yamato dynasty?