I sort of intended to overlap D'artagnan with Amaterasu: first thing is that I couldn't get any fire element weapons other than Yatagarasu's SSR gun (which is very great) so I had no choice, and yes below that is the chance of securing the -atk and -def lest D'artagnan's debuff misses. Other minor thing is that I have no healer in my team besides Amaterasu.
I will use my current team both in the long run and in the upcoming union event.
Alright, now I think choice B's has better output. I've been getting tempted with Arthur's +20atk instead of Gawain's -20def before I ask it here. Thanks for the advice!