What if he is actually Rexus?

Amaund Motierre clearly changes between the two quests involving him and the destruction of the Dark Brotherhood sanctuary, but Rexus stays the same. I also found it odd that Amaund would have a legionnaire as a personal guard and since he never leaves his side, it's odd when Rexus is in a completely different room when you meet Motierre for the second time.

Motierre also goes on about how you've "saved the Empire"; perhaps Motierre/Rexus thought that murdering the Emperor was the only viable way to get the Aldmeri Dominion to act, bringing about another war. This could be possible since Rexus is dressed as the typical legionnaire.

What are your thoughts, fine people of the forum?

No. It's the only way to get the Empire to act.

Thats false Stormcloak Propaganda.

In other words, it's true?

Aye lass, that's what I like to hear!

The following statement is true:

The previous statement is false.