Diane, I would say you have a pretty good understanding of the fee structure and the way you'll be able to do everything in II.
Yes, it appears that if you enroll in the DC and continue to do the same amount of Marriott-to-Marriott exchanges through II using your lock-offs, then your $165 annual Club Dues will be less than the Marriott/II per/transaction fees you'll be paying if you don't enroll. But in any year if you simply use your one Week as a 7-day home resort stay, which would cost you nothing in transaction fees, you will still have to pay the $165 Club Dues to remain enrolled in the DC (similar to paying II annual fees even in years that you don't exchange.)
Yes, we who have enrolled are able to use the same II interface to search and process exchanges and Getaways in our new/corporate II accounts. There are no fees for Marriott-to-Marriott exchanges; the same II fees will exist for outside-Marriott exchanges.
As you know there are no guarantees that either II or Marriott won't increase fees at any time, and that includes whether or not Club Dues will always be less than per-transaction fees for the particular way you use your Weeks. But you're right in that if they each are raised proportionally then you'll be ahead of the game. If at any time you're not, you can choose to un-enroll from the DC.
Many of us who have enrolled our Week(s) have not purchased additional DC Points but have still managed to use the 800 PlusPoints bonus. You could try to add a couple days to existing reservations, maybe do an unexpected weekend away within driving distance, maybe offer a night or two as a gift to family or friends. The PlusPoints cannot be transferred or banked so their use is limited, but I'm sure we could help you figure out how to use them instead of having them expire.
The one variable that is most unknown at this time is the effect of the DC on availability of both Marriott and II inventory. There are quite a few threads going with folks trying to figure out if we're already seeing effects or will in the future. But my thinking is that if there are adverse effects on availability, enrolled Owners will be able to choose among more usage options to try to counter such effects.
Good luck with your decision. The DC works for some and not for others depending upon what's owned and how it's used most often. Generally speaking it makes less sense for owners of one Week who rarely exchange, and most sense for multi-week Owners who exchange often or want to take advantage of the less-than-7-days-stay opportunities.
It is difficult to understand everything so I'm glad I could help a little.