My understanding was that this sort of stock sale blackout is typically an internal thing to avoid insider trading liability, but if you've been laid off, you would no longer be internal and therefore no longer subject to the blackout. Is that not how it's structured here?
You are subject to a blackout when you have material nonpublic information. IME the blackout remains until the opening of the trading window following the end of your employment.
Kinda legally yes, but some mid-sized companies enforce a trading blackout by contract and with a broker-level feature on everyone just so that they don't have to do the work of telling everyone that has that kind of information that they do. Twitter probably does this because of how the LinkedIn thing is worded.
The former employees could transfer their shares to another broker and sell them with no issue if they think they are on the good side of the law, which they probably will be after severance.