I know you've been dying to hear how the whole office-chair debacle turned out. Well, I'm pleased to say that I ended up being assisted by not one but two helpful people at Crate & Barrel, and after more emails back and forth they ended up having a replacement height-adjustment mechanism sent to me. It arrived this week, and it looked identical to the one I received originally (they told me they have changed the design), but I was hopeful that this one would work. So I immediately set about dismantling the chair in order to swap out the old piece for the new one.
This is where the story gets embarrassing (and probably boring, but I figure I owe you all the details).
I got the base of the chair separated from the seat and began to puzzle over how to separate the height-adjustment mechanism from the base. Once they're pushed together they're designed to stay that way, and I'm not sure I would ever have been able to pull them apart. Fortunately, I didn't have to, because while I was fighting with it, I discovered something: it works. I didn't need the replacement part.
Here's why I didn't figure that out before: the handle on the height-adjustment lever says "Lift to adjust" right on it. So that's what I'd been trying to do -- and it wasn't working, because as I noted previously, there's only a couple centimeters' clearance between the handle and the seat. At least, that's why I thought it wasn't working. But it turns out the mechanism is designed so that the height adjusts when you push the handle down. I could see that once I had the seat unscrewed from the base. I'm embarrassed that I never stumbled on that solution, since I thought I'd exhausted all possibilities in my attempts to figure out what was wrong. But I was focused on the "lift" directive -- and since there's hardly room to get a finger in between the handle and the seat, pushing down never struck me as an option. With the seat completely removed, though, pushing down on the handle was a breeze, and so I screwed the seat back on (and put the replacement mechanism aside). It's rather awkward to push the handle down when the whole thing is assembled, but it does work. Hallelujah!
So, I am sorry the Crate & Barrel folks went to all that trouble on my account. I'd like to think it will save them some trouble going forward, since I suspect I won't be the only person thwarted by the new height-adjustment mechanism and its misleading directions (which, for the record, comes from a third-party manufacturer), but maybe I really am dumber than your average furniture assembler. I am glad I didn't have to return the whole chair -- especially since I don't see how I could have managed to take apart the bottom, at least not without seriously damaging it in the process -- and I am also glad that I can go on being a C&B customer, because I really do like their stuff. I was window-shopping in midtown just the other day. And the chair looks really good in our living room. The end.