Does the selling of bitcoin-backed shares directly affect bitcoin's price at all?
This will not have an effect on the price of Bitcoin, because when someone sells their shares, someone else will buy them, so the value of the shares will not fall. However, it is different if a company that has bitcoin-backed shares goes bankrupt or top-level management decides to sell some of their Bitcoin to save the company's liquidity, it may have a direct effect on the price of Bitcoin. And it also depends on how much Bitcoin they sell, if it's not too much, then it doesn't have a significant effect.
That's the good thing about this market. Supply and demand will always be there because a lot of people are seeing this as an opportunity to accumulate their satoshis. Besides, you need a significant amount of btc before it can truly create an impact in this market. So one stakeholder selling its btc-backed shares won't cripple this market. It is actually a good chance to collect their stash.