1) The cap is 40 max per attempt from my understanding (so if you hit it, you will not get anymore even with a buff)

2) I don't have a full understanding of this 5 stacks, but based on what I know its stackable globally. So if one person spends 1k coupons to max it, no one else can.

3) Boost is for everyone in the group.

Hope that helps!

~Jib