I am looking for advice on how to run a wheezy chroot from a lenny host,
in particular how to handle dev, udev, and X.
While it's not an answer to the questions you've asked, I just want to make sure you're aware of a project called OpenVZ which is sort of like Solaris zones, but on Linux (i.e.; you have virtual "containers" for systems, all of which actually share the same kernel, but are otherwise completely separate).
Here's the Wikipedia overview:
And here's some documentation to get you started if it looks interesting to you:
This sort of solution sounds more or less like what you're looking for, aside from your desire to run two different distributions concurrently (OpenVZ would not support something like that).
Good luck with your project!