Very helpful comments - thank you. I think I'll go ahead and upgrade the rim - I'll feel more confident about it with something stronger. And a whole new set of spokes makes sense too. But it sounds like the most important issue is tension - how can I ensure that they're fixing that and not just truing the wheel?
Any suggestions as to what I should expect to be done under warranty? I expect to pay for an upgrade, but it seems like with 160 miles on the bike, and already having bought one new tire because the wheel was out of true, I shouldn't have to pay for everything. I like the idea of replacing the rims with the DM21's... maybe they'll do that and only charge me the difference?
I think I'll like my bike once I get all the fit issues worked out (and I'm getting close), and I don't know if I'm over-estimating how much more I'll enjoy a road bike, once I'm fitter, for longer-distance riding, but it seems like a comfort bike isn't the best design for 30-40-50+ mile rides. I hadn't thought about the possibility of using a high-end wheel on whatever other bikes I get, but that's a very good point. I definitely want a "spare" ASAP, but it might make more sense for that to be a mountain bike and wait on the road bike... but that's a whole different issue.