Check out landform package by Lauren Schmidt. Not sure it has all of your topo stuff in there, but it has a lot.
Sounds like you solved the topo by polysurface, so I will skip that for now.
A brief search turned up some promising looking results if there is nothing there.
This topic’s initial post has some promising looking python for creating the sub-region from a closed loop: Creating a Subregion *then* using it to shape edit a floor
And this post has a good example of creating building pads: Building Pad creating TypeError: expected IList[CurveLoop], got CurveLoop - #2 by Tomasz
I recommend annotating these heavily when you bring them over (ie: literally write down what import clr does) by commenting at the end of each line. This will help you learn rather than keep you reliant on other’s provides code. Just add a # at the end of the line and type up what you want to do.
For the masking region: why not just use a filled region which is solid white? These are more flexible anyway - ie: when you want to make the say green they override, or if you want a crosshatch you can change the pattern. There is a node for that in the core Dynamo for Revit library.