AM r8 does 575 dmg per 2,5sec and frostbolt r9 does 515-556 dmg per, so AM has a higher base dmg.
As arcane u don't want crit anyway, but arcane do get 10 less resistance from talents, not to mention u need 4% less hit from gear.
Why does it scale worse with gear?
One warlock can just use curse of shadow aswell. What do u mean no increased crit and arcane missile dmg?
Why would u only enjoy the Val fight in bwl?
Well I can start with obv stuff like Cruel Barb, Ironspine's Fist (all 3 items from him is pretty good for plank) and Hand of Antu'sul.
Also I believe that Fist of Stone from Landslide in mara would be pretty good to change between for questing (depends on the proc %).
I would recommend getting Corpsemaker from RFK at 29, which will last you til Bonebiter from In the Name of the Light which I would argue is better than Ravager, then The Rockpounder or Stoneslayer from uldaman at 44, which will last you til high 50's.