Are you moving or selling your collection? If you are moving it, you are NOT transferring ownership so an FFL is not needed. However, if you are selling it and transferring owner ship to another person, then an FFL is needed. When you ship the guns to the FFL, your are transferring the guns to the FFL. Is he going to be expected to "store" the guns until your return? Do you plan to use the FFL to further transfer the gun to some individual in Idaho who will "hold" the guns till your guns until your return? How will you get your guns back when you return? The Idaho person holding the guns will transfer the guns to an Idaho FFL who will transfer them to an FFL in your state who will then transfer them to you? Do you really want to pay all of those transfer fees?
Move the collection yourself and make arrangements in Idaho for storage of the items. Preferably in a safe at a relative's home or in a safe in a bonded warehouse.