I think you play too many video games Mersa. If it was that easy, we’d see a lot more historical examples of that tactic.

You can get distance or accuracy, but if you tried sniping like that, you’d have to get close and you’d be overwhelmed.  A group of slingers can maintain sustained volleys which make it harder to rush them.

I guess that would really depend on the terrain.  You’d need good mobility and enough foliage to conceal your movements but not so dense that it keeps you from slinging.  You’d want rough terrain between you and your enemy so they can’t overwhelm you but you need to be able to reposition quickly. That’s a pretty specific set of circumstances that would be needed for your tactical approach, but I suppose it might work sometimes. 
As a kid, I used to play paintball with slingshots, and the tactics were completely different from how you play with paintball guns, because if you miss, your opponent could rush and hit you point blank before you reloaded. It made you choose your shots much more carefully... especially when the other side didn’t know your location. It was also a ton of fun