"If I were to click the 2D bitmap, and wanted the same point to be marked on the 3D model corresponding with the .bmp, is this where the interpolation is necessary?"

That's right. But, you're the only one who knows what that correspondence looks like. MVVM architecture models will help you choose a place in your code for that kind of computation, but, again, the decision process is yours to make. Under MVVM, the interpolation probably goes best in the ViewModel.

Congratulations; you have crossed into the realm of Real Computer Science, where Easy Answers Do Not Exist.

What it sounds like you want is called "Hit Testing". https://msdn.microsoft.com/en-us/library/ms752097.aspx

Fortunately, it is baked into WPF, including WPF 3D. This will mean that you have to use Helix/WPF rather than Helix/SharpDX. You will need to develop your own linear interpolations to interpret the results you get back from the HitTest() methods so that you can program the functionality you want against the click methods.

As design patterns go, MVVM is relatively advanced. You will very steadily become experienced programmers this way.

I'm interested as well. Right now I do this by inserting a UI element over the region I want to capture and preventing the region from being resized, but it would be nicer to take this off-screen.