If you make a dense-ish cloud that cuts across the Starlink orbits you'd eventually intersect them all if you could make the artificial debris field last It wouldn't require that many different counter orbiting fields to cover most of the orbits.
Not much else uses those orbits right now. Other comms satellites and surveillance birds are all higher up. The debris would in theory also clear pretty quickly and should be fairly contained so the cascade of additional damage might be relatively small too. Hard to know that without a huge simulation budget to see how high the shattered satellite bits might get tossed.
Yes but there's so many starlinks that you're going to get lots and lots of collateral damage to sats from allies and enemies alike. It's going to be a huge footgun.