The Hard Fork podcast had a pretty good episode recently where they interviewed one of the engineers on the project. They’d troubleshooted the spacecraft enough in the past that they weren’t starting from square one, but it still sounded pretty difficult.

They apparently didn't have an emulator. The first thing I'd have done when working on a solution would have been to build one, but they seem to have pulled it off without.

There is an fascinating documentary about the team that sends the commands to Voyager 1 and 2 called It's Quieter in the Twilight

100% they've got an emulator, they've had dedicated test environments since the moon landing for emulating disaster recovery scenarios since the moon landings, they've likely got at least one functioning hardware replica and very likely can spin up a hardware emulation as a virtual machine at will.

Source: I made this up, but I have a good understanding of systems admin and have a interest in space stuff so I'm pretty confident they would have this stuff at bare minimum