So if you use your action and set the Mobil arc the l the left does that mean you now have a front,rear, and side arc... that's crazy if it works like that

No. Your only active arcs are the primary arc at the front and the one wherever the mobile arc is pointing. You don't have a rear arc by default.

No, you have a primary arc that faces forward and a mobile arc that faces left. You can't shoot to the right or behind you.

The lines on the Shadow Caster's base are just there to show you where the mobile arc can be. The only arc that's actually there all the time is the shaded one: the primary arc.

Great thanks