I'm no pro but am seeing some great results recently, including spending the night with a girl i saw in the mall a few weeks back... Here's some thoughts about your questions based on my own experiences thus far.
I personally don't do a full 'yad-stop'. I usually get to the side of them, slightly in front, lightly touch their elbow and say 'excuse me', then sort of get a little but not fully in front of them. I find it a little more discreet. Personal preference though. I have yad stopped and yosha stopped and both were effective.
As for girls walking towards you. Let them walk past first. Maybe try and get a little eye contact in as well. That way, when you turn around and stop her it's a little more movie-esque; 'aww, he came back!'
I don't tend to bother with a time constraint or anything. Sometimes i blurt out, 'excuse me, very quickly... i literally just saw you blah blah blah....' That's about as close to a time constraint i've used. That said, if i've left my friend sat on bench, i might go for the close by saying, 'anyway, i should get back to my friend who i abandoned etc.... etc...'
A little tip i find really useful, and it works on the principle that one of the most important things you should be doing is monitoring your reactions and responses to the girl's responses, is to show a little humility. Sometimes if the girl is really startled and embarrassed, i'll say something like, 'i don't normally do this, im kind of embarrassed myself!' or something along those lines. Confidence minus humility can come across as arrogance. Good luck!