There’s no problem. Enjoy.
mommamia –
Everyone knows you’re going into McDonalds to buy a drink or to use the bathroom etc., so there’s no mar’is ayin. The other concerns you mentioned are not issues for various technical halachic reasons. Just a very basic explanation* to make it understandable – kosher and not kosher is dependent on whether or not you can taste it. That means that your McDonalds coffee can only become not kosher if there’s a good, noticeable not kosher meat flavor in it, which will obviously never happen. Without flavor, additives can’t normally make something not kosher, even if they do not have a kosher source.
*I am well aware that I am oversimplifying very, very much, and no halachos should be applied from this statement alone to other questions.