No they don't. Attractions have two lines -- FP and Stand-by.
Most attractions at WDW are mobility-device accessible, meaning disabled guests use the standard waiting queues unless their disability involves an inability to wait in a queue. If the DAS is continued, they'll likely send those with return times to the exits for expedited entry.
You can sue anyone for anything regardless of whether you're right or wrong.
No, the reality is that FP+ lines will not be operational for pre-booked FPs, but most will be available for DAS (disability access), child swaps and service recovery issues. Will be much like MFSR for the first six months of operation.
They do not want people going up the exits for a host of reasons.