Thanks Dave. I should clarify, I haven't taken the internals out of the shifter yet, just unmounted it, removed the hood and bottom cover plate. Before further disassembling it, I wanted to try the simplest cure first, by completely flushing out any old grease and dirt to rule out simple binding of the float mechanism. Now that I've done that, it's pretty clear that something else must be going on in there (though I still can't imagine what). My plan is to order the alloy spring carrier and g-springs, and go ahead with replacing those when they get here. And I'm gonna be really pi$$ed if that doesn't fix it.

In fact, I may order one of the cheapie Mirage Escape right hand shift levers for $70 from Jenson, so my husband can at least be riding this bike while I'm pulling my hair out over troubleshooting this shifter.

Yes, hanger alignment is great, cable tension checked and re-checked, every possible microadjustment has been tried, attachment is correct. Cable movement is smooth as glass.