1. There isn't yet a "forget all" command for peers, but you shouldn't worry about it too much - eventually the node will give up on the old peers once it has no success connecting to them. But we'll note this as a feature to add in future.
2. If you're using a version of MultiChain earlier than the latest one (alpha 26), the unspent output error might be misleading, so you should upgrade to get the real error message. But if you are on the latest one, it is presumably exactly what it says – the node's wallet does not contain the private key for any addresses which (a) have admin permissions, and (b) have some unspent outputs in the blockchain. You can use the commands getaddresses, listpermissions and listunspent to diagnose and verify this – please update us here.