Ladies and gentlemen... I HAVE LINKED MY RESERVATION! So when I spoke to the Swan in-house reservations team on Thursday I specifically asked if my issue could possible be that my Swan Reservation used my shortened first name. And that the
MDE account used my full first name. I was told absolutely not, that could not be the issue. And so I proceeded to of course wait the 24 hours like they said and my linking again failed... My Marriott account has always used my nickname, dating back 15+ years - I never though it to be an issue until today although truthfully I should've fixed it long ago.
Well today, I decided that this must be the issue. So when I got connected to in-house reservations at Swan, I again brought up this issue. And this time... the lady I spoke to immediately said "Yes, that could totally be your issue." She then updated the name on my reservation and said it should re-send my confirmation to Disney too. I also then called Marriott to officially update my name on my Bonvoy account. Well... I didn't have to wait 24 hours this time... It was <10 hours and boom, I attempted to link and was successful!
So in case this helps anybody else having linking issues... Make sure that your First Name and Last Name on both your Swolphin AND MDE accounts match! Otherwise, you will have issues linking a Swan & Dolphin reservation to your MDE account.
I'm glad I got this figured out as it seems this might be the easiest way to authenticate for early entry benefit. Before Early Entry came around, I had no real reason to care about linking a Swan reservation to MDE.