Ah, that is easy. I use those and, to be honest, they don't last long. What happens is that the little lip at the back of the cleat wears away and you find your feet coming out. The yellow bit at the front also disappears. They wear away when you put your foot down. Replace the cleats when there is no definite "edge" to the locating lip at the back. They will continue to work for quite a while, but you want positive clipping.
As for your second question, there is no need to rush getting clipped in on the "free" foot unless you are sprinting across traffic lights that are set for cars and will change before you get there, for instance. The SL pedals are weighted so that they hang at the right angle, so just drop into clipped when you are good and ready. It is a wide platform, so you still have plenty of drive. Don't be in a hurry to clip in, and likewise, don't be in a hurry to unclip either. Have a look round, hover a little and then decide whether to go or to unclip. You have more time than you think.