my question as well -- can shorts just default en masse, preventing a short squeeze? i mean as far as i can tell, there's nothing stopping a fund (i.e. corporation) from going bankrupt and defaulting on all its obligations, while its owners walk away with all their personal assets (and assets in other businesses) in tact.
The shorts can't default en masse, because the shares they were lent - those were borrowed by their broker from somebody (such as us). If the people that sold short default, then there are multiple protections in place for those that lent the shares in the first place.
The most obvious protection is that the shorts must daily put enough cash (it's all handled automatically by their broker) into a 4th or 5th party account that enables the lender of the shares to receive 102% (that's the number I remember - it's at least 100%) of yesterday's closing price in the event of a default. That cash isn't in the short seller's account, it isn't in any of the actively involved broker's accounts, and it isn't in the share lender's account.
The other backstop is that as a share lender (I was one a year or 3 back), I don't lend shares directly to a short seller. I lend my shares to my broker, who then lends them to the short seller. If I don't get my shares back, it isn't the short seller that's defaulting on me - it's my broker. Brokers are really REALLY allergic to defaulting on their obligations. It makes it sort of hard for them to do business.
SO, brokers have the cash backstop that is adjusted daily (lets assume 100% of the value of the borrowed shares for ease). And then, the broker has an additional margin requirement that the short seller needs to maintain. I figure with as volatile as Tesla is, that margin is more like to be 50% than 30% - if anybody knows the actual margin requirement any of the brokers are imposing right now, that'd be helpful. So if you're a short seller and have borrowed $100k worth of shares, then you're maintaining a daily updating cash balance of $100k (updates based on end of day share price) AND you're maintaining another $30-$50k as margin (which the broker uses to transfer cash back and forth to that cash account) and as additional protection for the broker. My guess is that as a short seller, you're probably working with something closer to $100k worth of margin in my example so you're not being margin called constantly.
The brokers that borrow and lend the shares are very VERY well protected from your scenario (short sellers defaulting and walking away from their obligations). If a short seller did go bankrupt, the cash account would be used immediately by the broker to buy at market enough shares to return the borrowed shares to the lender. Along with as much of the rest of the margin value in the short seller's account to get those shares. And the broker has all of the authority they need to sell off as much as they need to in the short seller's account to keep themselves and the share lenders whole.
Bankruptcy for the short seller happens sometime afterwards.
If short sellers somehow pulled off the scenario you're describing, it means that they also pulled companies like Fidelity, eTrade, (any broker from whom you can borrow shares for the purpose of short selling), and other brokers into bankruptcy with them. I trust Fidelity et. al. to protect themselves thoroughly and completely from allowing something like that to happen.