IMO, a strike would mean instant liquidation. But then again, abrogation of contracts, if it happens, will probably cause a strike. Accordingly, it looks very bleak right now for UAL's employees.
Legal? Nobody really knows. The RLA greatly restricts the ability of airline workers to strike, but if a court tosses the contract, the employees probably become employees at will, which would allow them to "strike" if they wished.
Order them back to work? Hard to say whether the employees would "have" to work. Contract abrogation isn't like a typical airline strike situation.