The occ. deals double damage set bonus stacks with empyrean weapon aftermath, meaning you can do double and quadruple damage.
Twilight dagger's 100% proc rate does not prevent the double damage from proccing. (tested by single wielding twilight dagger with haste samba enabled and seeing double damage procs)
The AF3+2 pieces do not need to be equipped when doing /ja "x samba", but they need to be equipped while you're attacking. This means it's okay to use your af1 headpiece to increase the duration of samba. (tested by using samba with 0 af3+2 pieces equipped, swapping to af3+2 equipment, and seeing double damage procs)