1. The devices may still be in DCR, so they will need to be deleted from Common Services > Device and Credentials > Device Management. Then when you run a new Discovery, if the devices no longer appear in other devices' CDP caches, they will be gone from the Discovery report.
2. To view devices by their hostname, you have to pick the correct name resolution scheme in Discovery (e.g. resolve by loopback), then make sure the proper IP address is both forward and reverse resolvable. That is, the management IP must resolve to a hostname, and that hostname must resolve back to the same IP. You must also enable Discovey to update DCR.
As for the credentials, LMS offers a feature called default credentials. You can configure these under Common Services > Device and Credentials > Admin > Default Credentials. Then, enable Discovery to use default credentials (check the box). Then, every device which is added to DCR will inherit these credentials, and thus you shouldn't have to configure any other credentials for RME to be able to fetch configs.