I'm just trying to figure out how the dates work... I vaguely remember reading somewhere that it's something like, for example, in 3E 300, the 3E would mean Third Era, and 300 would be the year. But, just for clarification, is this right? I've searched around and couldn't find anything.

Also, different from my first question, when did recorded history start? As in, recorded by a mortal. The earliest I've found is around ME 2500, the approximate date that the construction of the Adamantine Tower begun.

Yet again, a completely different question, and more for clarification. The Aldmer were the first race on Tamriel, and originated in the Summerset Isles, am I right? Then some of them went to explore more of Tamriel, and eventually ended up as the Mer that inhabit Tamriel today.

Yes.

Recorded history started at 1E 1:
The kingdom of Valenwood is established by King Eplear of the Camoran Dynasty. This is the first written record in the Tamrielic Timeline.
The foundation of Adamantine Tower belongs to pre-recorded history, but somehow we know approximately date.

There are different theories... But for sure Aldmeri did not originated in the Summerset Isles, and most likely there was beast races in Tamriel before Aldmer.

The Aldmer originated on Aldmeris (Old Ehlnofey) which is believed to be kind of like a supercontinent like pangea.

http://www.imperial-library.info/history/merethic.shtml