1) The occupation lasted decades and ended thirty years ago, like many occupations in history these where dark times. Things do change, though, especially when leaders change.

2) Yes, they are just outside the borders on Cailan's request and they are waiting because they were turned back. Good thing about invading armies: You can easily stop them in their tracks by telling them to stop at the border. They just do that.

3) As of RtO, we learn that Cailan and Celen were planning a permanent alliance, this was postponed because there was this Blight problem. Celen tells Cailan that the Wardens assured her that this is a Blight and that they'll have to deal with it first. Celene is obviously not so inclined to invade rather than resorting to other means (like a political marriage).