The useable range of the clutch was too small, and worse, it happened while the tension nut was way too loose.
Cover tape tension varied from good enough when just barely touching, to way too much torque, at just 1/4 turn more of the tension nut.
I modified the cover tape idler so the clutch slot is now two tori with both slightly askew, so the O-ring should have two high spots which engage first. I also reduced the spring tension on the O-ring as much as I could by putting it back on the bearing directly.
I also took a trip to Walmart to scavenge some more pen springs. Eight clicky pens for like $1, but the springs are stronger, longer, and worst of all, they don't like to stay in-line when compressed like the one I had before. I'll need to plan for sucky springs I think, so I'm modifying the design so the spring is coaxial on the connecting rod.
Moving the connecting rod will be a good deal of work though.
In the meantime I've been iterating on the slot cover, trying out different bending amounts lengths, etc. Don't have a perfect one yet.