Boy did Katniss get lucky!
How did she know that Coyne was going to be standing directly behind Snow at the execution? It wasn't as if here was an obvious seating area that the new "interim president" would naturally be standing. And what was her plans if she wasn't?
I guess expecting logic from a young adult novel is a little too much. To accept the plans that work simply because they worked. No matter how much they relied on EVERYTHING being JUST RIGHT.