Every ATCO at eurocontrol works at Maastricht. No choice, no any other place
Not 100% correct. it is also possible to work in Brussel, Luxembourg or Bretigny after qualifying. Once aquired the skills can be put to use in other departments of Eurocontrol. This can be a temporary or permanent move.
I really though that when you reach this step it is pretty sure you succeed...
This is the real stumbling block for many candidates. At least one member of the board will be a controller who wil be assessing how you will function as a team member, your dedication, even the way your mind works. There's very little you can do to prepare for this except learning that when you have dug yourself a hole it is time to stop digging further.