RE: [CONNECT DgnPlatformNet] Windows Form Z-order

That is correct, must inherit from Bentley.MicroStation.WinForms.Adapter in combination with AttachAsTopLevelForm(addin, true);
There is a problem with Bentley Winforms showing in the visual studio designer, so I created a custom control that will toggle back to the regular form when in design mode. (Not sure if there is another way)

Also, I have noticed issues with maximizing the window, you can override WndProc and handle the maximize state on your own.