Will a Bitcoin ETF allow ad hoc creation of btc?
Futures make their money by shorting stock, borrowing stock high, selling, then forcing the market down and buying back to return the now less valuable stock and pocketing the difference and cheating real owners of the stock.
75% of the US money supply has been created since 2002. That's borrowing tomorrow's money to spend today.
Can a Bitcoin ETF cause more than 21 million btc to be created?
Short answer: No.Long answer: Yes, it may happen but not like you are suggesting.
There is no way to create more than 21 million BTC. That's a fact.
However, financial institutions are getting into BTC now, and they might start to fractional reserve their bitcoins, just like banks do with their Fiat.
They will not creating more bitcoin out of thin air, but they may sell an ETF of 5 million BTC while they only hold 3million BTC, for example.
Hal Finney has already foreseen this. Look at this discussion:
Actually there is a very good reason for Bitcoin-backed banks to exist, issuing their own digital cash currency, redeemable for bitcoins. Bitcoin itself cannot scale to have every single financial transaction in the world be broadcast to everyone and included in the block chain. There needs to be a secondary level of payment systems which is lighter weight and more efficient. Likewise, the time needed for Bitcoin transactions to finalize will be impractical for medium to large value purchases.
Bitcoin backed banks will solve these problems. They can work like banks did before nationalization of currency. Different banks can have different policies, some more aggressive, some more conservative. Some would be fractional reserve while others may be 100% Bitcoin backed. Interest rates may vary. Cash from some banks may trade at a discount to that from others.
George Selgin has worked out the theory of competitive free banking in detail, and he argues that such a system would be stable, inflation resistant and self-regulating.
I believe this will be the ultimate fate of Bitcoin, to be the "high-powered money" that serves as a reserve currency for banks that issue their own digital cash. Most Bitcoin transactions will occur between banks, to settle net transfers. Bitcoin transactions by private individuals will be as rare as... well, as Bitcoin based purchases are today.
A bitcoin ETF is a Bitcoin Backed up bank product. I think it is safe to say that it might be already happening in some funds. There are a lot of funds buying bitcoin now, such as grayscale. Do they actually hold all bitcoins they say they hold in their funds? Probably yes, but we cannot be sure (at least I cannot now)
This is why it is much better to buy bitcoin directly, instead of using an ETF (which has a financial institution as an intermediary). If you buy bitcoin directly, you know you have a fraction of those 21 million.