I had a multiple entries schengen visa issued by german embassy.  My first port of entry by air was switzerland. I was admitted to enter the schengen states through that port of entry. I once travelled by road from london to german by road. I had a german schengen visa too. My first port of entry was france. I was admitted into the schengen states through france without asking for my itinerary. I think at the port of entry they know that with that visa you are allowed to enter the schengen states so they will not deny you entry simply because you have sought to enter through a state different from the issuing state. All you need do is explain your circumstances.