My 2 cents here, I mostly agree that Explorer from Win7 has been severely crippled, and the one from XP is vastly superior in many aspects, for both explorer and taskbar/desktop.
But I don't think that trying to run the XP explorer.exe is going to be a good direction. It has been designed to be very OS-dependant, probably calling undocumented features all over the place (your ntdll call and the new "listview" DirectUI control in Win7 being examples of that) so that they would be difficult to run on older/newer versions. It has been just built so tightly-coupled to the OS that migrating it is difficult.
My personal solution to this are Classic Shell and 7 Taskbar Tweaker, both of those replace and fix fundamental features removed/crippled. I know using external programs is not ideal, but I don't mind them as long as they do a good job, and looking at their footprint they don't seem to cause and harm (2MB memory combined and almost no CPU usage). Combine with a few registry hacks to remove the useless libraries, disable full row selection and I now have a pretty functional explorer, running the native Win7 one.
Even if you manage to run older explorer versions on 7, there is no reason to suppose that programs will immediately have problems. Remember that by doing that, you're replacing the shell only, while still running on the whole Win7 kernel. We may argue about programs that tamper with the shell (extensions and the like) but all others, at least from the beginning, we don't have any reason to think they'll break.