Vintage MONO Cartridge Question ?

I don't think anyone would argue with you that using a mono cartridge to play a mono LP is superior to using a stereo cartridge to pay a mono LP.    And no one would argue with you about WHY this is true.  I also agree with you in principle that I would rather use a true mono cartridge (one that cannot respond electrically to vertical displacement of the stylus) than a stereo cartridge wherein the two channels have been bridged internally so as to produce a mono output signal.  Some might argue that using a preamplifier with a mono switch is identical in effect to using a bridged stereo cartridge to produce mono, but that's another can of worms.

Anyway, all I can do is to suggest you investigate modern true mono cartridges like the Miyajima, EMT, and Ortofon products.

A mono cartridge whose cantilever can deflect vertically can still be "true mono" if it creates no signal in that vertical motion. Such cartridges have the stationery coils aligned to respond only to vertical motion. The vertical compliance protects your stereo records from a brain fart when one might forget to swap cartridges before playing a stereo record.

And yeah, I know, the OP plans to have a dedicated mono rig where that "can't" happen. Still, just because a mono cartridge has 4 output pins doesn't mean it's internally a stereo cartridge with strapped outputs. Besides, how would that work? Wouldn't you need diodes or a circuit similar to what's in a mixer to prevent the two cartridge channels from feeding back to each other and creating noise? 

What such a cartridge *can* easily do is be an internal Y-adapter so the mono signal is sent to the left and right signal pins so you have true plug'n'play mono to your sterero outfit without need of an external Y-adapter to send the mono signal to both stereo channels.

BTW, from what I've read, the DL-102 may have a more limited bandwidth because its output is pretty high (3 mV) for a MC. This makes the coils on the cantilever bigger and heavier.

OTOH, that AT-3MONO/LP cart is HOMC with 1.2mV output, plenty for  my MM input. It has a conical stylus, which (in my experience) is evidently wide enough (.60 mil) to work well with mono pressings on both sides of 1962.

If you want a fancier Audio Technica, there's the AT33MONO, which is an LOMC (0.35mV) with slightly wider (.65mil) conical stylus. 

A mono cartridge can have vertical compliance.  The Denon 102, and many of the Japanese mono cartridges have vertical compliance.  What is important is that the cartridge contains a single  coil only, and does not generate signal from vertical movement (well there is some, but it is negligible).  A stereo cartridge, adapted for mono by coil alignment, or internal connection generates a vertical signal.  This signal introduces distortion and phase anomalies that are audible.