These all sound like the same sort of problem, that the system has got itself stuck on some previous reservation and refuses to let it go ("Let It Go, Let It Go" ... sorry!)
You might want to try creating a whole new login ("A Whole New Login..." STOP it, Andre!) for the BoG site, using a different email address that you have not used before on Disney sites. If that works, try to enter your current 12-digit hotel reservation number using the new login page, and
hopefully it will give you the right dates and allow you to make a booking. Hopefully anyway, sadly this is not an exact science. Don't worry about this being separate from your normal
MDE account, there is no need for the BoG site to match with anything except your hotel reservation. Good Luck!
Andre