I think that the key to success in knowing what essential services and exceptions should be maintained lies in having both parties, employer and union, involved at the bargaining table.
We regularly do this when we negotiate collective agreements. No one has any interest in leaving a business with defective heating, failing to conduct health and safety rounds or doing anything else that might endanger the survival of the business. Don't forget that there will be a follow-up after the negotiation, after the dispute. Generally speaking, the parties talk to each other so they can reach a settlement, and they agree on who they do and don't want to keep on the job.
If the parties can't reach an agreement, they may appeal to the Canadian Industrial Relations Board to resolve the dispute. Consequently, the board must be adequately staffed. Having been involved in many negotiations in recent years, I know that parties often have to wait months to be heard before the Canadian Industrial Relations Board.
Resources must definitely be allocated in the event of a dispute, but you also have to trust in the parties in place. Everyone has an interest in having a business stay healthy and enjoy smooth sailing after a dispute is resolved.