Backup copy is designed to sustain RPO it have, so copy only 1 RP per interval you set.
So if during 1 Backup Copy interval you have more than 1 RP of Backup job, only one of them will be copied.
If you want more points, you could make shorter interval of Backup Copy job. Please make sure that length of interval is sufficient to finish synchronization.
Forward Lookup key is designed to look RPs only created after start of interval. Say you have hourly backup job and daily backup copy started at midnight. Without that key job will copy latest point from yesterday, with the key it will copy first point of today.
About warning in Veeam One - it is generated if during interval you stopped with Sync Now there were no restore points copied, so you don't need to use Sync Now, new point will start copying itself. If Backup Copy already synched, warning should not be raised.
About "object is not disposed" - do you see that in powershell log file? If so, it will be great if you could open a support case - that should not happen, however usually that message in log is not dangerous and should not lead to any bad situation.