Re: who should play supergirl
Mischa is the best option I've seen from all the listed suggestions. I do hope, though, that they use the older origin story, and none of that 1990s protoplasm crap. As for the city, if she's born on Kanodr, Argo City, wherever; i don't really care so long as she's a full-blooded Kryptonian like Kal-El. The 1984 "Supergirl" film had its faults, but the origin was cool, so doing something similar would be a step up in my book.
If I remember correctly, Kara's parents were named Simon Zor-El and Allura. I also read somewhere that Simon was Jor-El's brother, hence the "cousin" aspect.