Good news - I forgot Rosie2167 was not continuing on after Event One, so only four leagues will have the extra manager assigned, not five. I still don't like counting on people to "not show up", but I think it's safe to say that losing 4-5 managers can reasonably be expected.
A bit late here, but since there is no bonus points for winning a division or wildcard this year, just put the 5 extra into a 6 team league--maybe give the winner 4 extra points.
A good suggestion, however, the "bonus points" this year come from playoff wins. I'll keep it apples to apples and set up 12 team leagues only, to keep things simple and square.