RESOLVED - Thanks Orlok, for helpful comment "Oh, make sure you assign "hold secondary mouse" not just mouse 2nd as that wont work for movement." That did the trick (in Beta). Essentially, where in other games would typically just have to add "secondary mouse button" for forward movement, in Arma III, also have to add "hold secondary mouse button", as Arma III differentiates between the two. And, of course, make sure neither is assigned to anything else.