In #2, as residential properties are not usually completely fungible, it would have to be an "IOU this exact house".
As a result, I think there would have to be something more like this:
1. Alice owns a property that she would like to keep, but will not be using for X months.
2. Alice's default course of action would be to lease it out for those X months and get rental income.
3. Bob offers an alternative: Alice sells to Bob, who promises to sell it back after X months at the same price, while also paying the expected rental income for X months, plus an additional cash incentive.
4. Bob signs a note promising to give back that exact house after paying Alice the rent-equivalent every month for X months, and an escrow agent secures the note by putting a lien on the property equal to its current value plus Y%.
5. Bob can now attempt to short, by selling to someone else for the current value, and putting up Y% in cash as margin, to clear the lien. Bob also has to insert a panic clause giving him the option to rescind the sale by paying the buyers that Y% before the X months elapse.
6a. If the market value drops, Bob re-buys the property and transfers it back to Alice as promised. The escrow agent returns Bob's margin and the difference in sale prices.
6b. If the market value rises less than Y%, Bob buys the property back and transfers it back to Alice. The escrow agent returns the remainder of Bob's margin.
6c. If the market value rises more than Y%, Bob is unable to repurchase the property with pre-secured funds, and Alice might be pissed. The escrow agent could hand over all the cash to Alice, but the property itself would be worth more than that. To avoid this, the escrow agent will activate the panic clause immediately, using the escrowed cash, whenever increase in value approaches a fixed fraction of Y%.
Note that if Bob does not sell the property, the terms of the note and security instrument make the deal indistinguishable from a lease. Note also that there is so much friction in the real property markets that it is just a hell of a lot easier to short shares in any public company that owns a lot of developed land.