The simple plug + knob is the right solution. It’s the most resilient.

The auditors should have their own devices separate from the temperature control to monitor them. All it needs is a few sensors, temp, humidity, etc.

No need for complicated cloud this-and-that nor air-gapping a grocery chain. Keep it simple.

I'm not sure about freezers (I guess there was no monitoring) but when I worked for a large UK supermarket chain in the 2000s this was how the POSs worked.

When I first started credit cards were not taken as online payments, the system just stored your card details and the amount. That was sent to server in the back office, which collected all the data, then every night it would make a dial up or ISDN connection to the head office and send all the data there.