So you will most likely need to convert your license in order to act as pilot in command here in the states. In most cases its a matter of paper work and they give you a license at the private level, anything more I think you are required to do the practical exam, such as commercial. However this doesn't help you much because you don't have a Multiengine add-on, so basically they will convert it to Single Engine only and in that case you can't act as pilot in command of a ME regardless of if you have an instructor or not.
Finally, I actually visited Mt. Rushmore about 20 years ago so I have no valid information for you regarding buses.