Update; here it is resolved by doing a few things which I’ll describe below:
It turns out that Return cap was hitting both the Pipe and Right Bracket keys, depending on where it was struck.
You might see in the previous video the bright edges of the caps at the bottom; that’s flash, or a thin edge of plastic from the molding process. For most caps I just carefully used a fingernail to burnish that down to a round smooth edge, reclaiming a fraction of a mm and reducing the light caught at the bottom of the cap.
For the Return cap, I sanded all the bottom edges, and dug-in a little more at the top and top corners. I think it’s 220 grit that I have, it could stand to be a little more fine but it works when combined with something like cotton or even just a finger to burnish any plastic fuzz.
With the Pipe cap, it turns out the whole thing was off-kilter. The alu plate doesn’t have anything to hold that one in place, and the Boba bottoms have thin-ish fixing pins, so the switch and cap are able to rotate out of place. Swapping-in a Gateron bottom helped enough to make a difference.
Those three things together eliminated the clearance issues, but I’m still not really happy with how the stabs sound. The clips let the wires move too much, and while various types of padding under the wire were all helpful, none could completely eliminate it with these stabs. I’ve decided to use Zeals on this build and revisit the OAs another time - I have a couple things I’d like to try that I think might make them viable for a silent build.