I was just about to make a post of my own, until I saw this. I have a small, relevant request:
Is there any way you could make it so that the arrow keys can move the selected ACEs around on the event sheet, for quick reordering? So, anything could be moved up or down, events/subevents could be moved left or right. If you moved this first subevent up or the last subevent down it would become a sub of the next event. There are probably a few other scenarios, but you should be able to get what I mean.
So many times I've been frustrated by the drag and drop reordering system - unless I'm doing something wrong, sometime it just doesn't want to put them where I want them. With a complex event sheet, sometimes it will just refuse to put events in certain locations.