The ironic thing is that this evidence is found directly inside the park itself, specifically the new museum outside the 50th movie.
The photo is from a newspaper article about the official grand opening of Disneyland:
So there you go. July 17th was invited guests only. The actual day Disneyland was open to the public was July 18th. Hopefully we can put this whole debate to rest now. And somewhere I know Darkbeer is grinning.