For full disclosure, I am the author of the previous post, as I used the Rhino 6 licence of one colleague to evaluate Rhino 7 WIP. I now keep evaluate Rhino 7 with a trial version.
That being said, I kept digging with no luck.
WPF, Winforms and eto samples cannot load a file.
This post mentioned limitations but without listing any.
What would be the current limitations of integrating the viewport in standalone dotnet application?
(object selection, load files, …)
This github issue illustrates well the issues with viewport control.