The list as I have explained before is so that the appropriate permissions can be set, for example, axis, allies, allied squadron leader, axis staffelkapitan. This is so they can view the correct parts of the forums and so that only leaders can discuss things in the officers forum for red and blue respectivley.
It is an administerial check list for permissions.
For numbers in the server its up to that squadron leader to ensure he has not exceeded his quota and is taking up slots intended for other people.
This brings me back once again to what I was explaining before, the campaign was designed around 99 players. We have 103 people saying they are comming to each mission. We all know that some will not attend, surely this number will be greater than 4% which we are hoping for so no one gets left out. I expect the actual non attendance to be around 20% minimum. I am sure Osprey there will be enough room for everyone.
EDIT to your edit. Yes as long as the "swapping" does not get out of hand its fine.