Citadels: Wishing Well card 'every other purple district' clarfication
I was 99% sure that that must mean "every purple district that is not this card", as you suspect.  But how to prove that for sure?  My first thought in such matters is to look at foreign versions of the cards.  Rules text that is ambiguous in English is sometimes much clearer when read in other languages.  

I haven't (yet) found a scan of a foreign version of Wishing Well... but this French page, (which contains a comment by Bruno Faidutti the game designer himself, so it must be fairly respectable!) seems to suggest that the text of Fontaine aux Souhaits is along the lines of "autres batiments violets" counting for +1; and I think you'd be hard pressed to translate that as "every second purple building".

(link: http://www.auteursdejeux.com/forum/viewtopic.php?id=707)

I'm going to keep looking for card scans I guess, but I'm pretty sure we're en route to the right answer here.  (What language was Citadels first published in, does anyone know?)