I have to applaud you on a good post. That's a lot how I imagined influence, twisting people to your ways, be it light or dark. One thing that annoyed me was the way that their alignment changed nothing, outside of their appearance and force power cost. The handmaiden, even though she's down on the bottom end of the scale, still preaches being nice to people. But then, fixing that would require a lot more dialogue, as you say.
I'd like to see influence affecting people a lot more. For instance, if they REALLY dislike you, they may just decide to up and leave you, or even attack you. This would require more hirable characters, BG style, but would be worth it. A lot like when Minsc would attack you if you didn't save Dyanheir in BG1.