100 points at HHI will cost about $1000/year. 50 points at RIV = $419. so you will bay almost $600 more per year to try to book wdw at 7mo - if you are looking at studios, you will have a hard time at many of the more desirable resorts. At what point does the cost savings in the initial purchase get eaten up by higher dues?
I think it is a perfectly valid approach if you love HHI and plan to go there often, but that is not something I would consider if my main goal would be to go to WDW. There are plenty of good neighbor hotels around WDW that would cost you less, and give you more flexibility, even at RIV's $419/year dues price. (50 points gets you 2 nights in a studio, for purchase price + $419/year vs. deals on hotels at $200/nt, with easier booking and cancellation and without the initial purchase price.)
ETA: I see you already own at HHI and Poly. If you have had good experiences using HHI as sleeparound points, then 100 more sleeparound points (as opposed to 50 at a new home resort) may or may not be worth an additional $600/year in dues. If RIV tugs at you, and you haven't stayed there yet, try it out. Even if the owner incentives for VGF2 go away, or go up a little, it might be worth the wait to see which you'd prefer at 11 months.