To date I have not seen CCV have any issue of unavailability at 11 months out. Thus if you buy now, you should be able to reserve late October 2019 time. Note, I assume DVD, as part of the sale, is going to bank those Dec 2017 points into the Dec 2018 use year, even though it is now beyond the usual banking date, because that will be necessary for you to be able to use them for an Oct 2019 trip.
Whether CCV will develop any 11 month reservation problem is not yet known. The issue noted above is how the resort is being sold. Those cabins have a huge amount of points applicable to them, but they may not have huge demand because the point cost per night is very high. As one can buy as little as 75 points, what is happenning is that huge nimbers of purchasers are being sold points applicable to those cabins but intend to use their points mainly for studios. The result is that there could be significantly more demand for the studios than number of studios available. Once CCV gets closer to selling out, that could possibly develop into a problem of getting reserations at 11 months out for studios, particularly during DVC's high demand season that runs from late Sep to marathon weekend in Jan.
As to what is the best use year, the issue noted above is one of potential cancellation. Normally you can bank up to all your points in a use year but only if you do so by the end of the eighth month of your use year. For a Dec use year that means Dec 2018 points have to be banked by July 31, 2019. If, for any reason, you have to cancel that late October reservation in August or any later, you will not be able to bank those Dec 2018 points and will have to use them for a reservation that ends no later than Nov 30, 2019, or lose them. If you have a Dec use year and go annually in Oct, that is a cancellation risk you would have every year. Thus, many recommend that one purchase a use year that begins shortly before the time one usally goes to WDW.