The game is constantly polling to see what players are doing and will scale things up or down as needed. If you're hitting an enemy once every 5 min the game doesn't consider you as a participant and it won't scale. Yeah if people leave an area it will be downscaled, enemies become weaker, less enemies spawn, etc.
If you really want to do a specific DE you're going to have to push it to that point, doing whatever that entails.
The events aren't random, the zombies will attack the fortress again at some point, but it's not so cut and dry. The events don't just start over on a timer, events are persistent and the outcome of an event then affects the world around you. If those zombies attack the fortress and succeed they'll push in and kill everything, fortify it, they'll keep moving out to conquer other areas of the map which spawns new events from that event. If YOU succeed in killing the zombies, the native wildlife will come back in, the zombies might retreat to nearby areas to plan a new attack, new events will spawn because of your success and you'll push along the branch in that direction just as you'd push along the branch in the other direction if the zombies took the fort.
I say branch because events aren't so black and white, success in an event might spawn a couple new events in the area, success of failure of those new events might spawn more events and so on until it reaches an end point in the branch. Failure from the same original event will spawn new events, and those spawn new events, etc. Then events can be affected by other events or other variables such as weather, time of day, etc. So events aren't one dimensional so you're really not going to know what's going on in the world at any given time, the world is far too dynamic to be predictable.
---------- Post added 2012-01-22 at 11:44 AM ----------
You can start participating again as long as the event hasn't concluded by the time you get back. If it does conclude prior to you getting back you'll get credit for the participation that you did while you were alive.