It would be difficult to try and hook Underworld into V:TM... In all honesty, right next to Blade and Buffy, Underworld is easily one of the worst vampire films I've ever seen.
You would be right in saying she seems to only use Celerity, in fact short of some "Crawling along the roof" thats done, thats the only vampire-esque power thats used in the entier film... Gun-Fu not really being anything I'm familar with.
As for her sect, Sabbat or Cammerilla. The group shes with seems well funded, has Elders in charge, upholds the Masquerade and works independant of each other... It sounds very Cammerilla to me. However, as you pointed out, their lack of laws in some other departments makes them seem a little more Sabbat. I'd be leaning towards a very lazy Cammerilla.
I'm not sure about her Humanity. The fact that she still belives she feels "love" suggests it might be higher then we first suspect, and if we put her in a V:TM setting then she wouldn't have to kill in order to feed from humans... In fact, that shes willing to switch to blood that doesn't come from a hot, living mortal vessal suggests that she might be around the 6-7 mark.
As for clan.. Ah... Hmmm... I don't really know what to say. She doesn't display any kind of individuality or real emotoin or quirks through the entier film. That might be the script and acting, or it might... As you suggested... Reflect a low humanity. Maybe shes a clanless vampire, hence why the others don't seem to like her very much, but are more then willing to use her as a pawn.
Your English was excellent, better then mine in most cases.
As for good Vampire shows, I suggest Ultravoilet. It's really quite awesome... The TV show, NOT the movie!!!