Depending on the circumstances, it could be construed as constructive dismissal. Creating unfavorable ultimatums allows companies to 'lay off' people without having to pay severance.
Well if the employer is suddenly requiring previously remote workers to come back to the office, they are in a sense firing them. Or perhaps more accurately "laying them off" because it isn't an individualized thing.
Yes, and if you have a union you should talk to them about unemployment protection.