As a PP mentioned, the end of May can be as hot as anytime in June. In fact, sometimes the beginning of May can be brutally hot at WDW. Just something to consider. Check out last year's temps and see what you think. Because I'd be surprised if your dates in May were the same price as dates in June.
As for renting DVC points, although it may seem like a wonderful idea, keep in mind that if you needed to cancel the reservation or leave the trip early for any reason, you'd still be paying for the entire stay.* If either of these problems occurred and you booked your resort direct through Disney, they'd be very accommodating. I've left trips early a couple of times and Disney did not charge me for the nights I wasn't there.
I personally would not want to spend two 15-hour-plus driving days, but maybe your family is okay with it. The drive home would be the worst part for me. And rope-dropping the day after arrival will be tough.
*In the case of some DVC rental agencies, if you canceled early enough, you'd get a % of your money back. Even this seems too risky to me, but plenty of people rent DVC points regularly, so not everyone feels this way!