Sick: Use it or lose it. If it's some kind of PTO arrangement, it might get treated all or in part like vacay but that would be complicated.

Back Pay & Vacay: Fed guarantees protection up to a certain $ cap. Additional protections vary by state law, but in many places it will be treated as a priority, secured debt. But that's assuming there's any actual money left... If all the cash is gone, you might get pennies on the dollar a few years down the road. FAA will not allow airlines to operate without any cash reserves (Mx, safety), so one can assume a major airline will still have cash on hand when they file.

401k: It will go down just like if you quit or got fired. You keep your contributions, and any vested employer contribitions. Non-vested goes to the company.

I would *assume* that your union is researching this stuff wrt state law, and will inform everyone if it looks like it's happening. If that's not a good assumption, maybe ask your reps about it.