To follow-up, I solved my problem yesterday. First, I'll describe the solution, then how I found it and why I think it worked.
My solution was to re-apply Mac OS X 10.5.1 Update by downloading it from:
http://www.apple.com/support/downloads/macosx1051update.html
After restart, I could use all keyboards and mice simultaneously, and the relevant pref panels worked as expected.
The reason this works is that a clean install of Leopard followed immediately by a full Software Update process seems to result in some (in my case, about 20) files with corrupted permissions which cannot be repaired due to a SetUID problem, as reported when repairing permissions through Disk Utility. Some of these apparently resulted in failures such that the topcase devices could be deactivated, but not reactivated or configured to remain active. Re-applying the updater fixes all but one SetUID warning, and (whether coincidentally or not) also allowed my internal keyboard and mouse to behave normally.