"my next question is: is it even possible to win 1.4b (hitting 200m with x7 multiplier)?"
My guess is yes based on the fact you showed that somebody apparently won 10x 200 mil gp, which should be under Double Quintuple Dragon Chest, I suppose.
I know for sure mutipliers don't apply to gear, so you won't get 7x or 10x Lucky Chaotic gear but the prize multipliers apply to gold pieces rewards.
BTW, although the person who won 2 billion gp is really lucky, I know somebody who is a lot luckier than him. That person got 2 combat pets dropped in a single Nex kill. The chance for that to happen is probably worse than 1 in 100 mil or even 1 in 1 billion.