I would think someone would be happy to lend you the number of shares you have blacked out for a large fee, with a contract you release that to the lender afterwards. Something like "we lend you 75% face of your blackout shares now, in return we get all your blackout shares when they're released."
Most of these anti-insider trading policies prevent all of the obvious "one clever tricks" by prohibiting entering into options contracts, lending, borrowing, shorting, executing futures transactions, etc.
If you could find someone to lend you cash (based on the totality of your financial situation and personal credit), that's fine. It's not fine to borrow someone else's shares and short them during a blackout.
Do they contain penalty provisions beyond getting fired? They're meant to protect both the company and the people from the slightest risk of insider trading allegations, so in most cases violating the policies isn't actually illegal insider trading.
Buying options is completely orthogonal to insider trading. Options contracts are not allowed because it's written in the employee contract and that's because companies don't want you to be engaging in stock activity that goes against the company. But not because it's illegal. If you're no longer an employee, you can buy whatever you want.
If you have MNPI you can't buy or sell anything, including stocks or options, even if you don't work for the company.
I don’t recall what the terms were for TWTR but I do know my current employer prohibits using these shares as collateral for a loan, because you may default and be forced to sell them in the blackout period.
They can’t if you own them. Obviously the creditor can’t collect until the blackout is over but that can be written into the loan agreement. Those shares are your property. Insider trading is a sec thing, separate.
You can just short Twitter stock from any brokerage in the US. That is the exact same thing. As a non-employee, you can do that without issues (unless you have MNPI at which you can't do anything).
Edit: Per below comment, don't do this :)