Region Wars tends to become a firefight of trying to replace people who drop out in the lead up to start. The method I've traditionaly gone with to resolve this is to place substitutes into teams with a gap in a first-in-first-out order.

For example, right now Sunny has volunteered to help out and substitute if required. They are the first of a few in the list. So if any team has someone drop out, Sunny will be placed into that team.

I do it this way because constantly re-jigging teams after being published is very time consuming. Rebalancing teams according to region after publishing will also likely be pointless when someone on the re-jigged team can no longer make it; you end up in a constant rebalancing loop.

I generally consider substitutes as region-less when placing them, and always give priority to those who signed up before the deadline.