Thank you @dwette
I have previously addressed similar questions and concerns on the Steve Hoffman forum.
https://forums.stevehoffman.tv/posts/37109436/
https://forums.stevehoffman.tv/posts/37825839/
https://forums.stevehoffman.tv/posts/37851830/
To add some background to the above explanations, the reasons why mono cartridges exist in both true and ersatz forms have mainly to do with manufacturing and component costs.
A bridged or summed "mono" cartridge is far more likely to be encountered when the underlying design is fixed coil (MM, MI, MP, MF). The underlying reasons lie in how the fixed armatures (also called polepieces) of such cartridges are made; as stacked laminations punched out with high-hardness, high-strength dies (similar to how the laminations of small transformers are made).
The orientation of the fixed armatures to the moving magnet (or moving iron) determines whether the fundamental architecture is stereo or mono.
Altering a fixed-coil cartridge from stereo into _true_ mono demands a revision to the armature shape, which requires the fabrication of another punch die of different shape. Such dies are fairly expensive, and that cost will be multiplied by two; one set of dies for the stereo armatures and a second set for the mono armature. (Note that the high cost of the dies is justified because it makes the cost of the individual laminations cheap.)
Technically, dedicated stereo and mono armatures could be done, and would eliminate the need for signal bridging / summing. But due to the die tooling investment and component cost factors (1000 pieces of a single type of component is cheaper than 500 + 500 pieces of two types of components), I am not aware of any fixed coil (MM, MI, MP, MF) mono cartridge that has a dedicated armature of different shape from the stereo models.
It is far more economical for the cartridge manufacturer to simply sum the outputs of the stereo channels together, and label the result "mono".
The situation is different for MC mono cartridges, since with the fore-and-aft magnetic circuits that are typically used, the orientation of the polepieces has no bearing on whether the cartridge is fundamentally stereo or mono. What matters is the orientation of the moving coil bobbin to the stylus. If the bobbins and coils are oriented at 45º, the cartridge will be stereo, if the bobbins and coils are oriented horizontally, the cartridge will be mono.
Since the bobbins can be made as part of the (moving) cantilever assembly, the cartridge designer can simply orient the bobbins at 45º for the stereo models and twist them horizontally for the mono model. This will require separate cantilever and coil bobbin assemblies for the stereo and mono models, but as long as this is done, there is no need for bridging / summing.
Of course, if the cartridge manufacturer is insistent that their mono MC cartridge must use bridging or summing, nothing stops them from orienting the coil bobbins at 45º, and for the mono version, summing the two channel outputs together. But in practice, this would only be done if the component budget is so low as to permit only a single cantilever and coil assembly, which must be shared across both stereo and mono models.
hth, jonathan