Thanks for sharing all the learnings and experiments - really great to see your usage of the volume tool and helps us understand where the confusing parts are.
For the area calculation you first asked about, @RemotelyPossible is absolutely correct - the area is simple the 2D area of the polygon - we don't take elevation change into account at all. If you need to get the true area of a sloping surface, I'd suggest drawing a couple of lines along the slope sides which should show you the elevation profile (which @RemotelyPossible again nicely linked in his post a couple of days ago), which can help with the true area calculation. Depending on the complexity of the surface, the blender option might be simpler. One point of caution re the blender route: our 3d model .obj files are not georeferenced / scaled to real-world units, so you may not be able to get an accurate area measurement from them. If either of you have actually gone through the process it would be great to know if it does work out.
For the take off elevation question, again @RemotelyPossible is correct - we reference our elevation model to the flight take off altitude. The caveat is that gps altitude readings may be off by 5-10m, so we don't always see the take off location sitting at exactly 0m elevation in the model. For really accurate elevations you have to either use GCPs or RTK gps.
For the volume measurement tool, we set the base plane for the calculation using the elevation of the points in the polygon you draw for the volume calculation itself. In your example above, we therefore used the elevation of the points you drew around the jungle gym to define the base plane. The volume cut and fill values will therefore change significantly depending on where you draw the polygon. Hopefully that makes sense...
Thanks again for all the detailed notes on your investigations and posting the links to our documentation - we love to see the product being used like this! Let us know if you still have questions or difficulties getting the information you need and we can find a way to help you out!