'A search for X11 on my system only pulls up the pacakges I've downloaded!' - are you sure 'X11SDK.pkg' and / or 'X11User.pkg' do not exist(s) in the '/Library/Receipts/' folder? If either one or both do, delete it or them.
'(Note that X-Code Tools, but NOT X11, is installed, so perhaps it's causing a conflict somewhere?)' - no.