The acmgd.dll, etc... you reference in the ObjectArx folder are just the declarations or signatures. There is no implementation inside the functions but allow the compiler to see the functions names, parameters, and return types.
That is why you set Copy Local to false because AutoCAD loads them at startup and when your plugin loads and it does not try to load them since the assemblies have already been loaded.
So testing code is not really going to tell much, and even testing a dll set to AnyCPU should work on either platform.
Even if your not making a call to a function declared in one the Interops do you still have it referenced in assembly?
VS2010 by defaults sets Embed Interop Types to true and Acmgd references the Interops so that might be the cause?
I think you should testing a built dll to try to diagnose the problem since everyone's build could be different.