As you have said, the creation/redemption mechanism will enable fair price discovery. The inability of shorting the underlying doesn't matter here, because if the ETF can be redeemed freely, an investor can buy the underpriced ETF and redeem immediately and sell in the spot market.

The only difference of this from actually shorting is the time. During the time of this operation, there may be adverse movement in the underlying price making the arbitrage too risky. It's true that fully efficient arbitrage may not happen.

But saying that there's no mechanism to maintain equilibrium price of the ETF is entirely wrong. If the ETF price is $60/BTC and the spot is $85/BTC, there's a huge incentive for investors to buy ETF and redeem and then sell in the spot market, even if it takes days. People who already have both BTC and USD in their hands can also quickly increase their BTC holdings, for free!

Therefore, the ability to short underlying simply doesn't matter, as long as ETFs can be created and redeemed fairly freely.