Is it like a normal entry list? Or is it still possible for those players to change their mind and enter another event? I'm not sure how it works now with the event giving ATP points.
I'm wondering about Mahut in particular, it means he won't play the RG qualies?
As far as I know, there are no withdrawal penalties for individual players but I would guess that the remaining two team members wouldn't be thrilled to have to play all the singles & doubles matches without a third player to help out.
There's no rule preventing players from playing the main draw of one event (Dusseldorf) & the qualifying for the next week's event (Roland Garros), but considering the distance between the two tournaments it's unlikely a player would commute back & forth during the week. Maybe Mahut is expecting a main draw wild card for Roland Garros? Or he will play Sunday/Monday in Dusseldorf & then go to Roland Garros qualifying?