Can I undo an OS update?

If you have a "Time Machine" backup you can restore to a backup of 10.5.5, but you'll lose all changes since that point (documents and data as well as OS updates and application installations).

Yup.

You can't 'Archive and Install' an older system onto a newer one, but you can 'Erase and Install'. You could the use Migration Assistant to load settings etc from the 10.5.6 system once you've installed the earlier system I think... IF you have a backup copy of it.

Might it be easier to troubleshoot the issues with 10.5.6?

Not true. You can Archive & Install, opting to save user and network settings. That's what it's for. Apple doesn't give everyone new disks when they issue updates.

My bad then! Apologies pianoman1976.

Does it enable you to keep all your software too then? Or do you still need to reinstall them?

AFAIK, it replaces all system software, but not Apple applications, unless they can't work with the older system software. Everything it replaces is put into the Previous System folder.