Don't know much about DF as I've never managed to make it work, but I think they say that there comes a time where a DF actually disturbs sleep rather than helping, I'm just not sure where that stage is.
As for whether a BF LO can go from 7.30pm to 6.45am, that would depend on the baby. (touch wood) DD usually manages from 6.15pm (start of feed, 6.45pm end) to around 5/5.30am ok, but you know your baby best. I guess you could give it a try and see how it goes?