just looking at the triggers and need to say.. I was here looking to see if you had figured out the rivers issue, but really you havent even figured out the tree or elevation issues, and just looking at the triggers I can tell you that it will not look very good when its done.
a better way I found to spawn trees involves a lot of units spawning and dieing, purely for collision and placement purposes.
my map is a large, shallow water map with 9 rects. each rect spawns 40 gulls of several differant types ranging from 2 tree to 10 tree(2 3 5 7 10) staggered on time to avoid lag, they all have negative regen and when they die they spawn the specified number of wisps and raise the terrain a set shallow ammount (they stack together to make hills) so when those wisps die they place trees, and after wards spawns another dummy unit, treasure chest model, the chest paints a bit terrain whichever (island snow sand grass) to the variable I have set for land type and has a random chance to spawn a creep.
The effect achieved is instead of random splodges, you have landmasses which are attractive and functional, and by starting my terrain as a shallow water, I can dig into and out of the water table, allowing islands and the like to be formed.
the system does require a bit of tweaking on your part, unit sizes ect, but if theres still a lot of interest I'll probably be releasing it as a resource after I complete it, it's currently the base of a survival map I'm making.