To remove a missing forwarder use the "rebuild forwarder assets" as you mentioned.
DMC > Settings > Forwarder Monitoring Setup > Rebuild forwarder assets
This will allow you to select a time range for which to exclude forwarders that have not checked in.
What about "Forwarder management" then "Delete record"?
Does rebuilding assets table clears missing forwarders in forwarder management (I wait 1h but still seeing old servers...)? If I remember but not sure, it worked in V6.