For your first questions, what you mostly likely want to do is restrict the population via a surface layer. In your population settings there is an option called "use density shader" Check the box, then click the button beside the empty field and choose "Create new shader > Other Surface Shader > Surface Layer" Then click the button again and click "Go to "Surface Layer 01" (or whatever it was named). In the dialogue box that pops up you can go to the Altitude Constraints tab and set what you would like your maximum altitude to be. For example if your mountains are 1000m high, maybe you want the trees to go up to 500m, but you want them to get sparse as the height increases, so you would set your maximum altitude to 500 and the fuzzy zone to around 150 or so. This should make it so that you get full tree coverage to around 350m and then it fades from there to 500m with no trees being higher than this.
As to the second questions, I am not all that great at making coastlines so hopefully someone else will be able to help out.
