I can't answer all of the questions raised here, but I can add some light at least. First let me describe my mono vs. stereo cartridges a bit further. I have a VAS Mono MC, .07 mil elliptical diamond on aluminum cantilever and the brother in stereo. Next an Ortofon 2M Black and 2M Mono SE, MM, both Shibata on aluminum cantilevers. Audio Technica AT33 Mono (2), one is 0.07 mil and one is 1.0 mil. Here is what I have learned. Use 0.07 mil styli on modern records, reissues etc., but not on old monos as they sound awful. Conversely, the 1.0 mil stylus doesn't track on modern records. So if you have a mix of old and new mono records you will need one stylus of each size. That is where moving magnet cartridges with user replaceable stylii can be handy. As I have already said a mono cartridge sounds best on a mono record. But of course the better the cartridge the better the sound.
Next, If you have a record that you are not sure is mono or stereo, start by playing it with a stereo cartridge. If the record is mono, the image will be centered between the two speakers. Switching to a mono cartridge will generally make the record quieter and more lively. For records that were "rechanneled for stereo" or some such nonsense, that means they are full of reverb and generally sound better if played with a mono cartridge.