Can I play my stereo records on a mono rig?

Just don't use a mono cartridge without vertical compliance (eg Miyajima models). You'll be fine with the Ortofon.

I have a Benz Ruby 3 converted to mono, and it sounds good enough that I sometimes wonder if I should use it all the time, but then I have only one ear and cannot hear stereo!

It should be fine for playing stereo records.  The basic design of Ortofon mono cartridges is the same for stereo and mono cartridges.  The suspension is the same so that the stylus is free to move in the vertical direction needed for a stereo signal even though this information is not produced by a mono cartridges.  There is no danger that a lack of freedom of movement up and down will damage a stereo record.  The shape of their stylus is compatible with modern mono records as well as those going back to the early 1950’s; this shape is aldo compatible with stereo grooves.