> This can be done safely with high voltage differential probes like the R&SRT-ZHD, but we don't have any.

Entry level differential probes are $300. Less if you shop around or buy used. Micsig makes a good starter probe that would be more than enough for 60Hz AC mains testing and it comes in a generic form that would have worked with this scope.

A lot of things can go wrong, some dangerously so, if you incorrectly probe high voltage lines.

I don't know why they got such an expensive oscilloscope and then proceed to cheap out on the most basic tools needed to use it properly.