A quick check suggests that offline credit card payments can "expire" if the network doesn't come back online within 24-72 hours, leaving the merchant with a loss. Not terribly reassuring.

Maybe where you are? But EFTPOS in Canada is done online. It is designed to. The bank interrogates the cryptographic processor in the chip and pin card over the network with each purchase.

I was unable to purchase anything with my bank card for more than a day during the outage mentioned above.

Merchant B won't be able to serve the unexpected surplus of new customers due to supply chain constraints and JIT delivery/lack of storage.

How does that help during a power outage?