Wait, Elenwen mentioned Motieree?
No she did not, but she says something that should set off alarm bells in heads; she slyly implies that her parties and other Thalmor parties within the Empire are a way of spreading Thalmor influence and securing interests since the rich generally enjoy the high life that the Thalmor can offer and ensure. Since Motierre comes across as the typical snooty and spoiled rich kid, I find it more than likely that he would be more pro-Thalmor.
Anyway, my hypothesis on Amaund's need for the Emperor's death to clear the way for a strong successor to guide the Empire out of it's declining fortunes is just that, a hypothesis.
However it is not one without merit if you consider his choice of words when the Emperor's death is confirmed. He basically told you that you may have saved the Tamriel.
Also, USEP appears to agreed with it. http://www.uesp.net/wiki/Skyrim:Amaund_Motierre
A couple problems with the points you mentioned:
1. As I mentioned above and has been implied in-game, the Thalmor are slowly winning over the nobles and elite of the Empire. It's highly likely that the super-rich and cowardly Motierre would be in their camp.
2. He says that you may have "saved Tamriel", but everyone says all kinds of things to the Dragonborn and the list of people who use the Dragonborn as a means to an end is rather long. Astrid uses you to restore the Dark Brotherhood, the Psijics use you to gain control of a powerful magical artifact from the College of Winterhold, Delphine uses you to restore the Blades, the Legion and Stormcloaks use you to eliminate the other faction, etc. What Amaund is REALLY saying is "you helped me eliminate someone I perceived as a threat and/or obstacle to a believe and/or objective that may or may not be good".