Based on review TUG and other web sites over 10 years I've never seen anyone really work the DVC system to the max like some do with Marriott so please do tell the secret.
I can't speak for others, but a typical resale DVC is priced around $60 per point, some may be higher and VB is the lowest (but has highest annual dues). Some contracts come with points and some do not, so assume only current year points (no banked or bonus points).
Assume about $5 per point per year in annual dues (varies from $4.50-$7.50) and assume rental rates of $11 per point.
Using the above real world examples, DVC nets about $6 per point per year.
That is an approximate 10% return which is way better than banks or CDs but not quite as good as your 12% return for Marriott example.
If you can buy DVC for below $60 and get free banked points and if you can rent for more than $11, then I think DVC may be better than your Marriott. Also, if you buy DVC for $100-145 from Disney, you will be upside down for a long time.
I did not know how to value the Marriott trip trades into Hawaii, etc, so I used your example of $15,000 to buy 2 weeks in Newport and netting about $1,800 per year on rent ($4,000 for two weeks - $2,100 annual combined dues). Also, I have no idea how easy or hard it is to reproduce that rental rate. DVC is pretty easy to rent and there always seems to be a demand. Also, the resale market for Disney is pretty stable and easy to sell.