Wow, nice summary KF.
The Harbie CAN be hit by darts, but it doesn't have any effect other than making a nice "thack" sound. Jugg is invulnerable, it seems. You can still destroy them slowly, though, but I think gas would kill the thing faster.
Forcewalls work fine on top of the bridges. Turrets slowly damage and destroy the darts, however. Mines work good as well.