CDO and Nonpositive Equity Questions

I have two questions about statements made in this video: https://www.khanacademy.org/economics-finance-domain/core-finance/money-and-banking/bank-bailout/v/bailout-2-book-value

At 6:25, the video shows a Collateralized Debt Obligation divided into an AAA-rated tranche, an AA-rated tranche, and an equity tranche, in order of increasing risk but also increasing return, and the claim is made that no one wants to buy the equity tranche. In my mind, the return on the equity tranche should self-adjust to make the market indifferent between the riskier and less risky tranches. Why would it have been so difficult to sell equity tranches that banks had to give up on the idea and hold them instead?

At 10:30, the video claims that a stock price goes to \$0 when a company has \$0 equity. Does this actually happen in the real world? The video seems to state that this doesn't imply you can pick up the stock for free, but that's what a \$0 stock price sounds like to me. Since the value of a stock is indeed its equity divided by the number of existing shares, how does this case play out in companies that have \$0 or negative equity?