vac..., You wrote, "I have also heard from audiophile friends that a mono setup brings out a deeper soundstage and sometimes better imaging which seems to run counter to the argument that the improvement is only related to surface noise reduction."
The language is a little confusing, because it could be taken as a claim that mono is superior to stereo in conveying soundstage and imaging, which is of course false on the surface. So, I think (or hope) what you meant to ask is whether using mono mode, via a mono cartridge on mono LPs is superior to using a mono mode switch, when playing mono LPs. That is the $64 question that I cannot answer, because I have not used a mono cartridge. This is where Elliot claims that a mono cartridge is superior to just the switch. Please see above where I described the mechanistic difference in terms of signal processing, between using a mono cartridge and using mono mode. (Of course, there is nothing safer than wearing suspenders AND a belt, so possibly using both mono mode and mono cartridge is best of all.) I can only say that I do hear suggestions of soundstage and imaging with just a mono mode switch that are not readily apparent when using stereo mode, with mono LPs. But Elliot may be correct that it gets even better with a mono cartridge. Sorry, but this boils down to listeners, systems, and subjective judgement.
As to which of those particular cartridges are true mono types, that is also a bit mysterious because manufacturers seem to try to obscure the methods they use to make mono cartridges from their stereo cartridges. (All 3 examples you cite are models that are also available in stereo.) I have a hunch that the Shelter mono is a bridged stereo cartridge. Ortofon is a bit tricky, because their literature is ambiguous, and they definitely try to leave the reader thinking their mono cartridges are true mono. Some may be true mono; I am pretty sure that some others are not. I never investigated the 2M. And finally, the AT33; I dunno. There are only a few brands where you can be sure of true mono. One of course is Miyajima. I think one of the Denons. Maybe others can help.