Valid counterarguments have been made. However,
given sites such
as
www.musicweb.uk.net/classrev/2000/apr00/scarlatti.htm
which I consider reputable and generally accurate-- as
opposed to someone's personal corner of the web which
lacks any editorial filters-- I personally would view
Christiana (but not Christiania) as valid were I a TD
resolving a protest.
My guess is that Christiana is
an obscure variant, probably introduced by an
Englishman.