Yes, they do disable all shorting for some tickers in rare instances, regardless of availability to locate.
Ok dude, I will confirm with IB, if you agree to the following terms. If we are right you will lie at our feet and say: "Sorry, you were right. I'm clueless." Agree?
Illiquid and thus inability to close out in liquidation. Combined with volatility of the name and inability for the firm to hedge your tail risk (and charge you for it)
Probably the same with all brokers. I'm sure their models tell them the probability of RGC exploding to the upside are slim at best. They're not their brothers keeper and they've probably allowed riskier bets than this before. I'm sure anyone holding the stock long have given their broker implicit instructions DO NOT lend out their shares to short-sellers.