Eagerly awaiting my Model01 (should be arriving today according to UPS) - but just a note about 3D printing with FDM. Yes, 4.5x2.2mm is small. But it’s not impossible to get rigid. In fact, PLA in general will not feel super nice as a material, compared to other options you may choose for rigidity.
Of course, ABS is a well known material and that may work. But I’d suggest PETG or nylon filament for printing custom keycaps. Both have a nicer feeling to the touch and much stronger layer adhesion. That should strongly mitigate the risk of broken stem pins.
But! Nylon is incredibly hard to print right. It’s strongly hygroscopic (moisture absorbent) and needs to print completely dry. It also tends to warp significantly.
PETG, on the other hand, prints almost as easily as PLA, you just need a higher nozzle temp and extra clearance on your first layer (or you end up gunking up your printer head).
Your solution about extracting the stem pin into a separate part is also a good one. If you want to bond the stem pin to the key strongly and permanently, I advise Cyano-Acrylate (CA) glue with the kicker spray (CA alone won’t bond well to PLA).
Very cool project and I may design some custom keycaps of my own after I’ve had a chance to get used to my Model 01