There is a setting already for allowing booted players to return to the game (a maximum of 999 time iirc).
For the other problems, mods can help a long way. There is (or at least was) a mod that allowed the host to instantly eliminate any player from the game, but that mod might have been removed due to abuse.
For forcing players to follow the rules, you could also look into one of the diplo mods. They force alliances between players and do a whole bunch of other things i dont know about. There are also mods that prevent AI's from playing (so that booted players can still return later, without their AI ruining the entire game)
Alternatively, you might want to have an alt or outside in the game that has 100 times more income than all others combined. The only purpose of that player is to get gifted in and eliminate any PE's.
You might also want to check out the following Discord servers (for advice and to find players):https://discord.gg/7KdEDKd3uG
- official Warzone diplomacy Discordhttps://discord.gg/TT2JZbvbbk
- unofficial Warzone diplomacy Discord