Usually we can "cheat" using the "made in" flag (one of the reasons why we added that field: for cars built under license)

The Ronda was derived from the Ritmo/Strada but according to Wikipedia was "redesigned just enough so that Fiat could not sue SEAT on the basis of patent infringement after 1980". So while it's still heavily based on the Fiat, it's changed enough to warrant a "Model origin: Spain" I'd say, same with the Marbella which is a redeveloped Panda. However, all the Seats with the same names as Fiats should have Italy as the model origin country.

How about having two flags for cars that have more than one country of origin? Such as the aforementioned redeveloped Fiat based Seats having both an, Ford Contour which was a collaborative effort between Ford of Germany and USA, Lada/VAZ 2101/2107/Riva which are all highly modified Russian/Soviet versions of the Fiat 124?

Yes that would be more correct. But would all the Spanish use agree?
The first "real" Spanish Seat is the Ronda I suppose?

http://www.imcdb.org/vehicle_144619-Seat-Panda-141-1981.html
http://www.imcdb.org/vehicle_56438-Seat-124-D-1972.html

I've noticed those Fiat-era SEATs all have model origin listed as Spain, which strikes me as completely inappropriate for them as they were essentially 100% rebadged Fiat models, so I'd say the model origin for them should be Italy and "Made for Spain".

I also think Polski-Fiat 126p should have Italy as its model origin, and the appropriate "Made for PL".