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I remember doing a demo at the WTC a few years ago and I was able to use and manipulate 3D Wolfram Language objects using Unity Engine in VR. I wonder how that worked...
Let me ask you another question. If you enable dark mode, and try opening Mathematica without that setting, does Mathematica open with parts of the UI in dark mode (and obviously not working)?
You mention "The front end, however, is essential if you want to interact with graphics, images, sound, etc." For manipulatable objects, this may be true. But there are certainly other ways of using those objects without Mathematica. See WolframLanguageForJupyter
I know you've already mentioned this, but I think Wolfram Workbench is a great tool, and sounds like it'll work well for what you're trying to do. A good in-between between Mathematica GUI and switching to .NET. What is your trouble in convincing them?