These numbers from the 2019 Marriott Vacations Worldwide Form 10-K Annual Report may help answer one of your questions. In the 10-K they say:
As of the end of 2019, approximately 185,000 weeks-based owners have enrolled approximately 292,000 weeks in MVCD’s exchange program since its launch in 2010, with more than 234,000 total owners able to use points.
The last time they stated specifically how many total owners that had in Marriott Vacation Club was in the 2017 10-K prior to the ILG acquisition, and then they said they had about 400,000 MVC owners. In 2018 and 2019 they only say that Marriott Vacations Worldwide has 660,000 total owners, but that includes the Vistana and Hyatt acquisition. They do disclose there are 187,000 Vistana VSN members and 32,000 members of Hyatt's point-based exchange program, so that leaves about 440,000 members who are either MVC owners or Vistana/Hyatt owners who aren't members of either VSN or Hyatt points. So, since we don't really know 2019 MVC numbers, I'll use the last real number we know of 400,000 from 2017, recognizing it may be a little more than that now.
So, based on these numbers, here is the math to estimate the percentage that have enrolled:
234,000 total owners able to use points minus 185,000 MVC enrolled owners = 49,000 MVC Points-only owners.
400,000 Total MVC owners minus 49,000 MVC Points-only owners = 351,000 MVC weeks owners
185,000 enrolled owners/351,000 total weeks owners = 53% enrolled
If my assumptions and approach are correct, I am surprised this number isn't higher. I wouldn't expect it to be 82%, but am surprised it isn't higher than 53%.