You cant mix the two APIs that I am aware of, so in my opinion the Interop solution is no good unless you are trying to do something simple.
Also, I have not found a way of passing an element from MDL to the DgnPlatformNET api.. Only for going from C# to MDL.
With Interop you could do MdlCreateElementFromElementDescrP(elementDescrP); but then you are stuck with an Interop Element.