[Chris Goodwin]

If you shoot an illusory wall, you will see an illusory version of your projectile bounce off of it and it will hit the ground. I would rule that the illusory wall would act as soft but complete cover, as regards whether the real projectile hits a target hiding behind it. Assuming it's not disbelieved, the illusory wall could be brought down by attacks, spells, etc., that would bring down the real thing, and in the same way.

If you tossed a rock at it, to you and everyone else affected by the illusion, the rock would appear to bounce off of it and hit the ground. The real rock would continue through, entirely unimpeded. It is very likely that the illusory wall would hide the sound of the rock hitting the ground on the other side, and any additional affects that may come from that.

If the arbalest bolt could damage the real wall, the real arbalest bolt would damage the illusory wall in the same manner, and potentially leave behind an illusory bolt. The real arbalest bolt would continue through unimpeded, as above.

Yes, that means that there's an illusory arrow, rock, etc., that the attacker could potentially pick back up and reuse. It remains illusory, though, and ends when the original illusion ends -- but as long as the illusion continues, it acts as a real projectile against anyone else affected by the illusion. If the attacker somehow finds their original, real projectile, that might be when they get a chance to disbelieve the illusion.