I could speculate on the specific answers to your questions, but none of them apply in this case, for the reasons Jere said. There’s never been anything like this before, so all the contracts have lapsed on both sides.
It would be virtually impossible for a producer to enforce an actor filling out any remaining “weeks” on a contract after the production had been shut down for a year and a half (otherwise I don’t think Karen Olivo would’ve announced that she wasn’t coming back to her show on Instagram), or to require anyone rejoin a stage show if they’d managed to get film or TV work in the interim. Similarly it would be hard for an actor to claim that producers were required to automatically rehire them after so much downtime.
Presumably, after a year and a half of no stage work, most actors who are available, and who haven’t left town or left the business will want their jobs back… and presumably, with so many shows needing to get up and running quickly, as long as the actors have kept their skills and their bodies in shape, most producers will want most of their same actors. There will be exceptions, of course (and we may even hear about some of them) but those will be the general guiding principles…. as every show and every agent basically works out deals that, however much they might be based on old agreements, will essentially be new contracts.