I don't let to control the shoot cause it will make the game too easy. For every 10 humans, you get a shoot (max 5) and I think it as a helper in a few situations to avoid the enemies that I positioned in problematic places. The fact it can bounce back is for some reason, to avoid shooting them from too near.
Some levels as you said had more than one path, some harder, some easier. The idea is to see "oh if I do this it will be easier". Or you can find a solution better than me.
The mountain level is tricky cause it is thought to force you to go fast. There are two points where the snow will fall over you and to avoid them is hard. But at the same time, I will give you some bad surprises to make you hate me as enemies. The good thing is that if you jump them, they will help you stop the snow.
As a side note, if you check in with directions the humans run, you will know where are some jellies. They run from the nearest jelly in the horizontal plane.