I just relived my whole nightmare through your description, Juan. I had very similar experiences and seem to have a similar network design. As I'd mentione earlier, once I re-installed, from scratch, the Extreme base station and ethernet-connected Expresses, the wirelessly-connected Expresses didn't need to be re-set. They retained their settings and worked fine, but everything on the Ethernet part of my network had to be completely re-configured. This was an absolute mess! I can't even imagine what the Apple techs must be going through fielding requests for help. I sure hope it makes them sit up and take notice before ever again releasing an update before it's ready. I completely agree with you: with all the complaints of similar issues, I can't believe that some of these problems didn't show up during beta testing!
By the way, unless I missed it, you didn't mention ever re-setting your dsl modem. After re-installing my main Extreme base station, I disconnected my cable modem and disconnected everything from it. I then re-connected the co-ax cable and the power. This forced a new Internet signal and my entire network, wired to wireless, is working as perfectly as it was before this horrible update. If you haven't completely reset your DSL modem, that may just take care of your DHCP issues. Good luck.