Just an update in case anyone was wondering... I mounted a dop stick with the stone onto the small engraver, and gave it a try with just 220 grit slurry in water. Sharpie marks were halfway gone within minutes, and the hand-cut preforms were now mating much more nicely. Then, mysteriously, the grinding stopped - the markings on the top half of the perimeter were just staying there, and the inner piece was not getting any deeper.
It turns the culprit was this bit of dop wax that was overhanging and covering part of the girdle. I ground that off, and just as I restarted the messy work, the stone popped off. I blame it on cold water I used to clean up the mess after the first round.
So I decided to try with CA gel adhesive on aluminum 10-32 "Chicago" screw (the barrel part) that fits nicely onto the engraver pin with a heat-shring tubing sleeve. The screw head was slightly domed, so had to grind it flat first.
This time I added a few drops of dishwasher rinse aid to the SiC slurry, seemed to help keeping it dispersed. Really the main issue when it comes to slurry was drying out soon after I would brush it onto the stones, and I guess some glycerol or another humifectant would be helpful.
Another improvement was placing a ribbed plastic liner under the outer stone to allow for used slurry to drain away.
Again, it was going great for a minute or so, the inner stone was working its way in, when it popped off AGAIN.
Of all the issues I was expecting when planning this character-building project, adhering stone to a stick was not on the Top 10 list.
I am only a millimeter away from calling this "good enough," but really getting sick and tired of cleaning up and trying yet another adhesive. Epoxy? Gorilla urethane? Not looking forward to it at all.
Once it's all nested in as I was hoping, I will epoxy the two pieces together, and finish making the cab. I get to enjoy this hobby project an hour here and an hour there, so might be a few weeks before I show the final result.