I've been doing a lot of these in my current go-round with Kettlebell Muscle.
As Dan John likes to say, "You can't think your way through a ballistic lift."
Keep it simple. Geoff Neupert's cue is "jump and punch."
As to your questions, here's my take:
1. Don't think about it. Use the leg drive to launch the bells out of the rack. Your arms shouldn't really leave your chest until your legs are just about locked out, otherwise you are just wasting leg drive. Inertia should keep the bells glued to your body during the drive--until it doesn't.
2. I think it depends on the weight of the KBs, the number of reps you plan to do, your purpose for doing the exercise, etc.
With lighter weights, you can balance the amount of leg drive and arm pressing to emphasize either. With heavy weight (or when your arms are fatigued), you may need every bit of leg drive you can generate just to get the weights to lockout.
Also, I don't think of the leg drive "assisting" the lockout. The leg drive gives you some momentum out of the rack, but the lockout is what your arms do. I guess this IS assistance, but I don't think of it that way; I think of the lockout as the "press" part of "push press."
3. Also depends on the factors mentioned in #2. With heavy weights and/or fatigue, you don't really have a choice.
A couple of other points I find helpful in my experience:
--The push press rack is different from the press rack. Keep the elbows glued to the body for the dip. Instead of thinking "spring loaded" as I do when cleaning into a press rack, I think "one piece," meaning the bells, my arms and torso are all one piece.
--Don't dip too deeply or too slowly. Keep it short and quick.
--Be very careful about dropping the bells back into the rack. It is very easy to jack up your shoulders if you just let the bells fall and don't absorb the impact correctly (ask me how I know). It hasn't happened to me, but I can also see it being hard on the back if your form isn't right.
Personally, I no longer even think of it as "dropping" the bells into the rack. I lower the bells with some resistance, just quicker and with less resistance than I would in a strict press, and use a knee dip to at the bottom. I also reset after every rep, not the "viking" style of immediately driving back up after catching the bells in the rack.
Hope this help.