It would be a court of law that would declare the contract is frustrated since everyone's interpretation of what is a frustrated contract would differ.
You have that right!!
I have commented previously that I think performance of these rental contracts has been frustrated by the closure of the DVC resorts.
I also indicated that in this case of frustration, money exchanged should be returned. Someone commented that frustration absolves the parties of any further performance of their obligations, and so nothing further need be done, but what has already been done (i.e. money exchange) is not required to be undone. I'm skeptical. I think that legal theory of frustration is applicable to longer term contractual relationships in which a series of obligations and payments, over months or years, are to be performed under the contract.
In the case of these rental contracts, there was only one single obligation (on the owner): to make available accommodation that the renter made payment to David's for, and which payment David already partially transferred to the owner. So I think this is analogous to making a contract to buy a painting, in which the buyer pays a deposit, following which the painting is destroyed. The contact cannot be performed. I don't think the law provides that the seller can keep the deposit. I'm sure this was a first year law example but maybe I am out to lunch on this!?
But...on further reflection, the analogy is far from perfect: the DVC resorts are still there, unlike the painting my example, so bookings can be made. It may just be much more difficult to get a booking, and it may need flexibility on both owners and renters side. Frustration isn't generally found if a circumstance makes performance of a contract more difficult or somewhat different from what the parties the contract contemplated. Food for thought!
It's all really just an academic question -- no one is going to court over this, and consequently no one will be forced to pay up if they don't want to or are unable to.
The advice to let your conscience be your guide sounds as good as any to me....