Hello Guillaume,
In my opinion, for the best interest of the game, you're rulling is the best you can make.
If player A doesn't show his cards, after player B mucked his hand, then you can retrieve players B cards, and continue the game like nothing happened, but it's not the case here. In this case, in my opinion you can't retreive players B cards and declared them alive because if you do that, player A has a big disadvantage at the game and he did nothing wrong, it's player B who's not paying attention who's turn it is.
That being said, player B losing only the 10K bet of player A it's the best rulling for this situation in my opinion, but I gladly wait for others to judge this situation.