As for new teams taking over existing teams records, perhaps something like this:
After a certain point of the season, let's say either the end of the 1st round, new owners couldn't be assigned teams with winning records. If a top team goes bot, it is probably fairer to let all the teams have to contend with its talent, rather than hand it off to someone who will have a good record, but no players.
This will tend to make it harder for owners who start late in the season from promoting during the bot cleanup. But there is no reason for them to expect for that to happen.
When a new league is opened up to active owners, all the records could be reset, or maybe set to 0.500. Teams would then have a sprint to determine who qualified for the playoffs. Once the playoffs have started, no new owner should replace a team that is in the playoffs.