The armsmaster doesn't have the Two Weapon fighter (melee) in his list, but he does have other talents that require two weapon fighting (melee) as a prereq. Is this errata or do I have to buy that as a elite advancement? Currently we're making it available, it looks like a simple omission.

Well, the only talent that requires two-weapon-wielder(melee) is Dual Strike at Rank 8. All the others require (Ballistic) which they do get at Rank 2. I wouldn't worry about it too much, and just let them Elite Advance it or put it on Rank 7.