"Then again, I don't see the necessity for the Cult of Supreme Being (indeed, I shouldn't judge him from a 21st century American perspective), but I guess that was one of his ways of achieving Rousseauian ideals."

I thought the same until I actually read The Social Contract. Then I realized how the idea of civil religion is completely reasonable and acceptable. Rousseau explained it really well.

Regarding your other questions, I can only agree with the previous posts. The Terror was a necessity no matter how harsh it sounds. People often forget the circumstances in France at that time. France was in danger in every way. So, as the French would say, à temps exceptionnel, mesures exceptionnelles. No matter what the revisionists might think of it...