IMO cooked sushi is ok. We've offered DS avocado rolls (loves them), cucumber roll (so so) and unagi (freshwater eel - wouldn't even touch it). DS loves Japanese cucumber salad and udon.
We're avoiding shellfish with DS .... so no California rolls for him!
If K has already had crab then IMO California rolls would seem ok to try. My general thought is that if DS has already had all the ingredients then I'm ok with letting him try a new food.
I think I remember that K has had shrimp? Then how about things like shrimp tempura rolls?
Soon after we offered DS avocado rolls, I asked my brother (who used to live in Japan and travels there for work very frequently) what toddlers in Japan eat. He asked his friends in Japan, and one said "For 14th babies in Japanese eat baby foods but any food is OK. From 1year to 1.5 year is the final period of baby food and the ratio of mother's milk v.s. baby food is 25% v.s. 75%. So your nephew can eat pickles and cucumber roll and udon."
and they also sent the link to a Japanese family's food diary that had all meals a LO ate between 8 and 14 mo. This is the 14 mo page.
ETA: DS also loved Chinese style chicken potsticker dumplings. And then we discovered that the ones DS likes - they use egg white to seal the dumpling and DS is allergic to egg