A new update to my OP. We had an owner update at Nanea over the weekend which actually went well (I will post more on that elsewhere), but the rep we were with said this multiple login issue is a major complaint they hear from other owners. His explanation was that MVW is treating the ownerships as 5 separate entities: MVC, SVC, WVC, GRM (Grand Residences by Marriott), and R-CC (The Ritz-Carlton Club). He said there is no plan to be able to merge these under one ownership access portal. When any of these ownerships are converted into club points, then you can see the converted club points in Abound. On a webpage he went to that is the currency converter to Abound, was a drop-down that required you to pick from one of the 5 ownership types above. I’m not sure if that calculator is accessible by any of us owners.
Still sounds odd, but that is what I was told.