A good trick is to unclip just one foot where it is possible that you might have to stop - clicking back in if it turns out that you don't have to stop after all. As you know engaging the second pedal is easy when the first one is already clicked in.

I am continually unclipping-reclipping my left foot in traffic, the only cost is that the left cleat wears slightly quicker than the right, not so much that it bothers me though.

In answer to your first question I fell off on my first ride on cleats - 0 mph - beside a bus. Plenty of amused onlookers.