One can not be freely substituted for the other if all that is claimed for the nature of each of them has any validity. The only way to logically engage in such free substitution is to acknowledge that neither deity exists. This means that any claims which invoke either of them are equally meaningless. Only if they claimed their beliefs to be inherently superior to mine would I take issue with them. The second category, that of fictional characters, would be easy to refute.
None have any support for their existence outside of their depictions in a book.
Clearly, he does. The fact that you can see the old man does not prove or disprove that he might actually have been responsible for the universe. After all, you are adamant in your defense of God while only paying lip service to any other deity. As for Harvey or the GGAS, it is intuitively obvious that they don't exist, and therefore, could not have created the universe. That is not intuitively obvious.