On the site it was decided that the Origin spot would be for "Spiritual Origin" so wouldn't all of the GM T-body cars (RWD) be origin Germany?

See this site: http://www.muenster.de/~mnickel/framee.htm

These are the ones on this site:

Kadett C:all [C] no link as the are not separated

Chevette: http://www.imcdb.org/vehicles.php?make=&model=Chevette&page=1

T-1000: http://www.imcdb.org/vehicles.php?make=&model=t-1000

Acadian: http://www.imcdb.org/vehicle_228165-Pontiac-Acadian.html

Isuzu I-Mark:1981-1984 only http://www.imcdb.org/vehicles.php?make=&model=I-mark

Gemini: 74-84 sold as Holden, Opel and Isuzu

Piazza: http://www.imcdb.org/vehicle_153381-Isuzu-Piazza-JR120-1981.html

Marajo: http://www.imcdb.org/vehicles.php?make=Chevrolet&model=Marajó&modelMatch=1&modelInclModel=on

Maepsy: http://www.imcdb.org/vehicles.php?make=&model=Maepsy

Chevanne: http://www.imcdb.org/vehicles.php?make=&model=Chevanne

Buick/Opel: http://www.imcdb.org/vehicles.php?make=Buick/Opel&model=&modelMatch=1&modelInclModel=- on

Kadett City: http://www.imcdb.org/vehicles.php?make=Opel&model=Kadett+City&modelMatch=1&modelInclModel=- on

I think thats all that we have listed on here

If the car is just a little based on it and not really a rebadged car produced elsewhere, we do not keep the original origin
What do you mean with the Piazza for example? Isn't it a fully Japanese car?

I think you are right about the Piazza (It seems that the information is inconsistent on the internet about this) I don't think it falls into this category.

as for the rest some of them are already listed as Origin Germany.

http://www.muenster.de/~mnickel/framee.htm

I think they are more than just a "little based" on each other but it is fine to leave them the way that they are already listed.