Was the Summerset Isles a part of the Empire at the time? I do recall reading that the Second Empire ruled all of Tamriel save Morrowind, but then there are people who are saying that the Altmer only became a part of the empire under Tiber.

Also who assassinated Savirien-Chorak? I know his father was killed by the Morag Tong who he himself use to kill Reman III and his heirs. The Brothers of Darkness seems to implicate the Dark Brotherhood but I heard his death isn't clear cut.

Reman united the human kingdoms of Tamriel, not the human and elven, I believe.

The PGE 1E mentions that the Second Empire sent ambassadors to the Summerset Isles, but they were only allowed into the capital of Alinor. The Elf annotator is disturbed to read even about that much "human trespass." The PGE 3E claims, "The rise and fall of Empires, the battles between men and mer, none of these touched the Altmer as a society." Based on these statements, I'd have to guess that the Reman Dynasty never took Summerset, or even nominally claimed it as their own. I don't remember what 2920 claimed, but I do kind of remember a statement to the tune of, "Morrowind is the only territory out of Reman's grasp now." However, 2920 is historical fiction, and when I first read it I found it so similar to the impressions I had of Tiber Septim's Empire that I didn't even realize it was talking about a different one until Reman III died without heirs and the Potentate took over (at the time, I had no idea about the history of Tamriel and didn't understand the dating system). As far as I can tell, the series is both somewhat anachronistic and highly romanticized, so everything we learn from it should be cross-referenced by every other source we have available.

And since 2920 is the only source I know of that explicitly claims that Versidue-Shaie was responsible for Reman III's murder, we can't really assume that that's true, either. We know he was assassinated, but no one knows by whom (or even if the Morag Tong was involved, IIRC). Edit: Fire and Darkness claims that the Morag Tong assassinated Reman III, as claimed in 2920; The Brothers of Darkness, on the other hand, claims that the Morag Tong's first-known murder was that of Versidue-Shaie in 2E 324 (The Brothers of Darkness does not explicitly deny that the Morag Tong was responsible for the death of Reman III, but claims that there's no proof of any murder by the Morag Tong prior to Versidue-Shaie's). Fire and Darkness also criticizes the 2920 series for its presentation of the Night Mother, but has nothing else to say about its presentation of the Morag Tong at the end of the First Era.