If I respond with duty to the clan to play an attack, then I use all my momentum (say for multiple, or powerful) would my opponent would still get the same bonus that I had got when I used the response?

Yes, because X was set when the ability was played so your opponent will have the +x to the next check floating.

The X is set when you initially resolve the ability and won't change. Whatever bonus you gave your check will apply to your opponent's next check as well.