Any chance you could post a few sample pics with EXIF data? I ask because it seems to me like your setup is fine.
I mean, yes, focus issues will obviously make it difficult to get a clean key, but I'm not sure why you'd be having focus issues when shooting from a tripod as long as the model remains the same distance from the camera. With the lights you have, I don't think any auto-mode would select an extremely bright aperture, so DoF shouldn't be an issue. Even more so since some images are over-exposed. Only thing I can think of is: are you perhaps shooting with IS on? But even that, while it might cause some blur, shouldn't cause over-exposure.
As for the bleed, 6 feet again should be fine. Only thing I can think to recommend is to lower (darken) the backdrop lights. (It doesn't have to be brightly lit, just evenly lit.) Oh, and it shouldn't be a reflective material, obviously. (Or you could try using Photoshop's "Decontaminate Colors" option, but I'm sure you don't want to spend that much money unless you have to.)
Sorry, hopefully someone else can be more help - Mark