1) As the world's second worst judoka (on a planet with 5+billion people on it, probability and logic dictate that somewhere, somehow, there's someone
who's worse at this than I
...but probably only one....
), my experience is that your left foot (assuming a right -reaping o-soto gari) must be past uke's left foot or your chances at breaking his kuzushi are considerably diminished.
2) My TKD grandmaster has a very useful variation in our self-defense one-step sparring. You block/redirect the attack and apply the o-soto conventionally, which leaves your right arm going forward - and immediately whip that right arm back in a down block against the kick from the ground that is heading for your blind side!
Finish with the technique of your choice....