Adding to and echoing pbacot's excellent advices, I hope this might also be of use.
I would stick with the native SU tools and get a thorough grip on what can be done with them. Have a look at some of the YouTube videos; Justin Geis's TheSketchupEssentials has a lot of good information.
There are, of course, many different approaches to terrain modelling but with the Sandbox tools in mind, using a combination of these could help rather than using just the one tool. Definitely keep your building footprint and hard landscape as separate groups or components. I have done some work on your original SKP and hope that it will be more illuminating than what would be a rambling commentary on what I did! Essentially, I drew the required outlines on a flat surface above the model and used the Drape tool to indicate where to cut into the terrain and re-build with the drop curb and sloped driveway. If you would like to PM me I will try and answer any queries.
Terrain Testing SB 2015v.skp
Dropped curb added to your original file
With hidden geometry turned on