Thanks. At the risk of forumscorn I took a crescent wrench to the hanger and gave it some leverage. Not sure I gained much, if any, but thought it couldn't hurt.

The RD is now dropping down onto the 12 cog about half the time, better than before, but not right yet. When I re-assembled I double-checked my limits, and held pretty good tension on the cable, in the small cog, while tightening. It's still not jumping inward very successfully, so I'll also be working on that tomorrow.

I'm hoping it's my inexperience with brifters causing my woes, and that I'll figure it out eventually!