No, it's the opposite. Internal memos are "discoverable" and we enter them into evidence in court all the time. Exception of course would be a memo from their attorney, which would be privileged/excluded. It also needn't be in a memo or even in writing: employees and ex-employees can also be deposed. I take no position on whether any such information exists, but if it does, it will probably come out quite readily. Disney is too smart to destroy evidence.
As for the signage, we can all debate it until we pass out from lack of oxygen, but this type of situation has been heavily litigated and there is case law as to what kind of signage is "reasonable". That's not to say it's black and white. It never is. But I would venture to say that if Disney was aware of the presence of alligators, the lack of specific warning and the presence of nearby sandy beach with lounge chairs makes things difficult for them. Not impossible, just not great. Attractive nuisance will be an interesting theory to watch as it generally doesn't apply to lakes, even man-made ones, as long as they "appear" natural. I know, law is weird.
Terrible, tragic event - the poor family will never recover.