1) To clarify, do the securities pay based on the first trade at 6.00$ BTC/USD or 4.00$ BTC/USD (a so-called "one-touch" option) ?
If I understand you correctly, yes - the first time there is a trade at at least $6, the long bond becomes entitled for 1 BTC no matter what happens later. The first time there is a trade at at most $4, the bond immediately expires.
2) By buying the short bonds, one doesn't truly short BTC/USD, because if the exchange rate goes to ~0 BTC/USD, the bonds are effectively worthless in USD.
It's an approximation which I think would be good for most use cases. It isn't for someone who believes the price will crash to 0 in the middle of the night. But someone who believes in a gradual decline (like the one from $31 to $2) - whether if it is because Bitcoin is going to fail, or because it is currently overvalued and things will get worse before they get better - can buy a short bond, roughly double his bitcoins, and then sell them or reinvest in a new shorting instrument.
3) Do these securities make the market more efficient? Suppose a USD-holding party thinks BTC/USD is overvalued and wants buy the short securities (thus shorting BTC/USD), they must buy BTC/USD on an exchange, thus raising the BTC/USD price. Therefore, this party has the opposite impact that they should to stabilize the market.
I think so. As a matter of general principle, I believe that for every action there is an equal reaction, and that it's impossible to make a bet on a market without it ending up affecting the market itself.
By way of specific mechanism, I as an issuer have a certain position I wish to achieve. If someone buys a short bond, he'll have to buy 0.5 BTC for it, but I will also need to hedge my position by either:
1. Selling 2.5 BTC, or
2. Issuing more long bonds and/or decreasing their price, in an amount sufficient for someone else to buy one more long bond - preventing him from buying 2.5 BTC.
Of course, this is all amortized over many trades, so is more difficult to see on a smaller scale.
The crux of #2 and #3 is the securities are BTC-denominated as opposed to USD-denominated, and the argument against denominating them in USD is obvious (regulation, etc.).
Right. It's an approximation designed to answer specific needs with as little overhead as possible - and it relies on other markets in order to ultimately transfer USD in or out.