Miller!
About the Device being rediscovered, in the delete Device section, check the Prevent Rediscovery.
Then this Device/Server will be at the excluded_devices.csv file at CA\Nimsoft\probes\service\discovery_server
You can check with CA Support help to figure out why this Device is still appearing, or just ignore and stay with it at the file.
About the Data Engine retention time, yes, after the retention period, will delete the Device and this Data.
If you want to do it manually, you can try by your own the scripts from this another community post:
Deleted Devices Qos-UIM
OR better this Tool:
Delete server/device in UIM via command line tool (v3.3)
Kind Regards,
Alex Yasuda
Sr. Support Engineer