Once a multisig wallet is created with a specific scheme, can a new participant be added, or does a new multisig wallet need to be made? The same question for if a participant can be removed?
No. Once the wallet has been finalized, you cannot change the participants. Should you need to add a new participant, you would need to create a new wallet with the old and new participants then transfer funds from the old multisig wallet to the new one.