You ARE swapping something when you use BVTC at 7 months, you just can't see it because the system does it for you.
You can either see it as the system letting you use "your" points from one resort at the other resorts, or as them converting "your" points into an equal amount of points at the other resort when you make the reservation at 7 months (and then converting them back if you decide to cancel). Either way the outcome is the same so it doesn't matter. In the example you had earmarked X number of points at RIV for a stay that year, but used them at VDH instead. This opened up X number of points of availability at RIV, which I was then able to book in the example. Again either way, the outcome is the same so it really doesn't matter. 7 month availability is there because owners at the resorts opened that availability by using their points to stay elsewhere, banking, or borrowing for a year. If everyone decided to use their full home resort points at their home resort at 11 months every year, the 7 month BVTC would be basically useless and there would be 0 availability anywhere but your home resort. You are swapping a stay at your resort for the number of points you used elsewhere into the BVTC system.
In my possible third party example, the third party company wouldn't be holding the points, they would be holding money, a deposit, etc, and yes one of the other users would be holding your reservation or the points you are trying to use. If done through a company with an agreement and deposits, up front, it could provide at least a little safety net, like the rentals do now. For example, if the other user breaks the agreement they forfeit their deposit on their own reservation and the company hopefully tries to make it right. Maybe they could hold a bigger deposit and once both your rented out reservation and rental reservation are complete you get part of it back. There are various ways it could work to make both users comfortable.