That's why liquidity and volume are essential to a quality exchange. If you enter that contract at $400 and you see bitcoin spot price jumping to $500 but nobody is trading futures so you can't sell it to them, you get screwed. Luckily at this point in 2016 there are numerous options for trading futures that are liquid and settle on short time horizons. So you can typically trade contracts and get in and out of them with decent spreads and reasonable fees.
As for why you should buy a put option instead of the asset itself, the answer is simple. By buying the asset itself, you can never profit from falling prices. With put options you can, simply because their value rises as the price of the underlying stock is falling. In addition to this feature, they offer the same kind of potential for leverage that calls options do, as described above. The price of put options is calculated in a similar manner, but with the important difference being that the intrinsic value is calculated as a predetermined price of the option minus the current market price of the asset – not the other way round as is the case for call options.
Unlike many commodity futures, Bitcoin futures are cash settled rather than physically settled.  Cash settlement is a relatively new development in futures trading, first introduced in 1981 for Eurodollar futures, that addresses the problem of how to settle futures contracts on things that are difficult/impossible to deliver physicially—things like interest rates, large stock indexes (e.g., S&P 500), and volatility indexes (Cboe’s VIX).  Futures physical settlement involves actual shipment/change of ownership of the underlying product to the contract holder but in practice, it’s rarely used (~2% of the time).  Instead, most organizations that are using futures to hedge prices of future production/usage will make separate arrangements with suppliers/customers for physical delivery and just use the futures to protect against contrary price changes.  In practice, the final settlement price of the contract can be used to provide the desired price protection regardless of whether the futures contract specifies physically delivery or cash-settlement.
Individual shares of the Fund will be listed for trading on [the Exchange] and can be bought and sold in the secondary market at market prices. The market price of shares will fluctuate in response to changes in the value of the Fund’s holdings, supply and demand for shares, and other market factors. ProShare Advisors cannot predict whether the Fund’s shares will trade above, below or at a price equal to the value of the Fund’s holdings. Differences between secondary market prices and NAV, the value of the Fund’s holdings, may be due largely to supply and demand forces in the secondary market, which may not be the same forces as those influencing prices for securities or financial instruments held by the Fund at a particular time. Given the fact that the Fund’s shares can be created and redeemed in Creation Units, ProShare Advisors believes that large discounts or premiums to the value of the Fund’s holdings should not be sustained.
Note that you could just keep bitcoin on CryptoFacilities waiting to make the trade so you don't have to wait to move the bitcoin you bought over. This is called see-saw arbitrage model, where you keep funds on both exchanges to avoid having to wait. This is fine, but you can't ignore that there is extra capital being used in the play, so it affects your rate of return and capital utilisation. We will not use this method, we will do a full, complete, legitimate arbitrage process.