Has anyone resigned and left before the 90 day period?
Do you mean in some sort of official way? Because most certainly a good few have simply slipped away forfeiting their Dubai possessions and PF contributions, etc. My personal favourite method is the at least two I know of that operated out of Dubai and then just didn't come back. Can't think of a better way to express to the company your opinion of them than THAT! Seeing as EK chooses to ignore the numbers of those leaving formally and then continue to abuse those who remain.
As far as getting permission to reduce the 90 days I haven't heard of anyone doing so.