Sign yy I have very often wondered this.
Theory #1 is something simplistic and vague about all members of the quartette being equal but some being more equal than others, and it being a demonstration that Jo is now bigger and more important than in her school days.
The other two are v context-specific wrt the timing of Exile -
Theory #2 is that she had already planned/written much of book #14, including Jo's conversion, proper wedding, and triplet-spawning, before the Anschluss made the book unviable. So this explains, potentially, why her central character marries off-stage, why it's never clear when Joey has converted, and the choice of godmothers could v easily be explained within this context - esp if they were involved in her conversion.
Theory #3 is that it might not have seemed politically advisable, by that time, to have had Austrian godmothers to English children.
I do feel that there must be some sort of explanation - particularly as at this stage all (?) of the European characters are Catholic, so it's unlikely she forgot - and two of the actual godmothers make their swift departures from the school and series at this time, so it's also not a question of EBD wanting characters who would be nearby.
I read Rosalie recently and don't recall anything about Dick's commission, but it's something I could quite conceivably have skipped over.
Emily, if I am ever busy again, that is the announcement I shall use - 'expecting a happy event'!