Unfortunately, once I get going with linguistics, I don't shut up. :) No reply is needed to this, I'm just babbling now and have to get this out or I'll be thinking about it all week.

Honestly, do not give up the idea that a universal grammar exists. It just may have dimensions to it we haven't considered. (Kind of like the English grammar rule i before e, except after c. You could set up a rule stating recursion occurs in all human languages except in those where the people have this other construction, or this specific view regarding time, etc.)

You can always use this important linguistic rule: When all else fails, set up a word class. (Kind of a cop-out, but it does come in handy.) You have two groups of nouns that follow completely different phonological rules? Set up word classes. You have two languages that treat recursion differently? Set up grammar classes and see what other research comes in. :)

All right, I'll stop.