I have been thinking of the shroud like a magnetic field, but an etheric field. It stops etheric energy from passing through, which is why you wouldn't pull your webbing inside the shroud. The webbing runs up/down/out the masts, beyond the radius of the shroud, and then spread out in the currents both air and ether. Once charged, the web interacts with etheric current, more strongly the more it is charged. Sails would be protected by the shroud though and, since the wind would also get through, would function. And yes dropping rocks or other projectiles would definitely get through the shroud.
IMO The real issue, as far as cannons go, is the potential differences between firepowder and gunpowder. In TAW it's firepowder which is so corrosive to the gun barrel, which could also make an exploding cannon ball a very volatile proposition. Old school muskets would have plenty of gunpowder left over in the barrel after firing a shot. If that is what is destroying the barrel, I wouldn't want a bunch of those rolling around in the belly of my ship. All the major advantages of an exploding cannonball would be drawback if firepowder is unstable and/or corrosive. (I keep picturing that old MacGyver oil rig fire episode with the nitroglycerin leaking out of the old dynamite.)
Ether is abundant and cannon crystals/bolts exist, so why risk blowing yourself up? This isn't our earth, basic chemistry is different. Maybe mineral deposits are super rare, too. Gunpowder, iron, tin for bronze, all of these have mining at the core of them; which could be a very dangerous game on a planet where all the wildlife is already dangerous before it is maddened by your blood. Just imagine the moles.