There is also absolutely no way for CMs to look at letters and know that they are actually from a doctor. Anyone can create letterhead and claim it's from a doctor and fake a signature. Even if you ask for licensing info, is WDW going to be able to get licensing info for every single doctor in the world?
As a side note, in most states there is absolutely nothing that says you need to hand over your photo ID to be copied or let them take your photo for your chart. My mother went and checked this with the states of both CT and MA (we live right by the border so see doctors in both states) and in neither state do they have any legal standing to make it required that they have any photo of you in your records. Both my parents no longer let doctors copy their photo ID or take any photos. They'll show their photo ID (I think that part is the part that is most likely to be a legal requirement), but they do not even hand it to the office people. They do, of course, let the office copy their insurance cards.