I have a 28/45 Pyro, that uses bows a lot and has one complete playthrough with ranged weapons only.
2. I think all DLC bosses are quite resistant to magic and elements.
In any case, don't use elemental arrows, they suck.
Lightning Bolts on Lightning Avelyn do a good damage, but since 1.06 +15 and Heavy do slightly more damage + you don't have to worry about resistances.
3. Avelyn is aweome, but you must know when to use it. It's bigger problem using it on Manus than Artorias, since Manus has faster lunge and leaping attacks, that can punish you while you reload it.
Ring of Fog works extremely well when you co-op with a melee build, because bosses seem to generally care about the closest threat, unless you're a caster. Casters tend to aggro bosses most of all builds, but, unlike most games, bosses in DkS can't maintain aggro.
If you're alone, this ring becomes useless. It will allow you to get one free shot on most enemies. Bosses ignore the ring completely when you're alone.
Gough's Greatbow does a LOT of damage, but if you go Str AND Dex, use Composite. With 27/40 your Composite will outdamage other bows. It's a little, but noticable difference, however, Composite fires faster!
With that in mind you can fire up to three arrows while Gough's does only one. So in terms of damage you will do more with Composite. I use Gough's only when I need to knock down, or stagget my opponent. And sometimes Pharis's when I need to snipe something, as Composite has short range and terrible aiming.
Co-oping with a bow is great, since you can shoot a lot of bosses from places, where they can't harm you. Like sniping Kalameet from a cliff, Iron Golem from the roof where the firebomb throwing giant is and more.
Have fun with this build.