All of it can be done. There are multiple ways to do it all.
1. Import png using file input, set "Resampling method" to fractal. Adjust roughness. Or do it the old way, import png, use blur filter to get smooth shape, combine with some noise, and erode. First way gives you more accurate results. Use erosion after to get realistic results and masks.
2. Answered in "1"
3. Yes, you can do it. Again, many ways to do it. All of them a bit hard to explain in text. Look up some "combining terrains" tutorials on geekatplay site:
http://www.geekatplay.com/world-machine-tutorials.php Good luck!