So why shouldn't they try to fix that mistake now? That is the part I am not getting here. What about them not having rebelled before now invalidates they rebellion? Seems like an ad hoc(it is ad hoc, right? I get some of those "ad" things mixed up at times) justification, to have some ground for condemning the rebellion that tries to break Skyrim away from what you yourself has described like so:
https://docs.google.com/file/d/0B6qpq3FrJbxjM3lqY2tqQ2lSRGs/edit
The Thalmor lost their invasion army and couldn't keep Hammerfell territory. While in better shape than the Empire, the loss at Red Ring seems to have been significant at their military might. Sustaining an invasion of a land with Skyrim's borders across half a continent wouldn't have been realistic.
As a child. Taking an oath as a child is not the same as taking an oath as an advlt, and, as far as I am concerned, no one should be forced to hold to an oath they took as a child, because you're not capable of making such decisions at the time. He was never given a choice in following the Way of the Voice, being sent to High Hrothgar as a lad. When the Great War broke out, he made his choice and that was to not follow the Way of the Voice.