As seen on 'Reaper', Nina was a demon who assumed human form when she was dating Benjy. The ability to take on that form was magical in origin, but where did she get the idea to assume that particular look?
I think it could be that she saw somebody else who looked exactly like that and decided she liked it enough to assume it for herself. As to who that person was, one candidate might have been an assistant district attorney in Dallas named Liz Traynor (currently seen on 'The Good Guys').
Nothing says that Nina had to be locked into staying in the Seattle area. There could be a thousand reasons as to why she had to go down to Texas. Probably thousands more, considering it's Texas - but don't go tellin' Bill Crider I said so.......
Just look into those eyes! You don't want to mess with her, whether she's Liz or Nina!
[Jenny Wade plays both characters.]