Day 18 is lockdown for all breeds of chickens.
It actually won't hurt them if you put them on lockdown a little early, such as day 16, or 17. Some people do it, I've done it once, didn't have any issues.
as long as your close to end they will be fine either way, I locked down at day 15 one time cause the eggs were rocking pretty hard and they still waited until day 20/21 to hatch and 4/5 got out just fine
Yeah, it really doesn't matter. The main point is that you have a way to add water without opening it, if you do it early. I routinely lockdown on day 17, sometimes 18. They don't have to turn really after day 12 or so I believe it is.