There is actually a bit under forests that says that if you do move any unfixed trees from the defined area of the forest, that you put them back where they were if possible, or as close as. So they've obviously considered how the trees changing location can effect line of sight.
Dreadknight isn't Fearless
That's quite funny, I've only just realised about MC's benefiting from area terrain so I hadn't considered going to ground. I have some Dreadknights available to my Grey Knights... so if I take one of those to scare the pants out of someone by deepstriking it into a nearby ruin... then they all shoot at it with what ever AP2 weapons they have, I could go to ground and get a 2+ cover save. Awesome.
might try that out next time. I know it would be an expensive bullet magnet, but its just a bit of fun. : )