Question:
Is there a way to host an MDI form from an ActiveX DLL in a Main application’s MDI Parent Form?

Answer:
Although I’ve never tried it, I don’t think so. Only one MDI form is allowed per program and a dll runs in the same process as the host program so it probably won’t work.

An ActiveX control also runs in process so you would have the same issue. You might want to try it in an ActiveX EXE which runs out of process.