Glad to hear you like the data explorer improvements.
The UnavailableException could be caused by a couple of things. The first cause would be that not all nodes in the cluster are up; if OpsCenter doesn't show any nodes down, that's probably not the problem. The second is that for a few versions of Cassandra, if there was a node timeout during the truncate, an UnavailableException would be raised instead of a TimedOutException. If the data size for the pdps column family dropped to ~1MB, the truncate went through and just timed out. (Sometimes, not having JNA set up properly is the cause for slow truncates.)
You can drop all OpsCenter data by shutting down opscenterd and the agents, dropping the OpsCenter keyspace, starting opscenterd, and then starting all of the agents.
However, that's probably not necessary for fixing your issues. The repair issue may be that Cassandra is legitimately hung, or there may have just been some lost messages between the agent and opscenterd. If you run nodetool netstats on the nodes that OpsCenter says are streaming, you can see if some parts of the repair never actually completed. If nothing turns up, restarting the agents on the streaming nodes should resolve the issue.