1. worked if not in light infantry mode
2. usually because they run out of ammo quickly and/or face the wrong direction
3. Happens to any other unit too
4. Funny enough every man must have his rifle reloaded before he can plant stakes
5. see 2.

Oh, and once their target dies or routs and if they are in light infantry mode, they can shoot all the way across the map too on their own unless you give them any orders.