I can answer a few of my questions myself after some more experimenting:
- PVID is the vlan which untagged packets who arrives to a physical interface will be "sent" to. That is how this packet will be tagged internally within the device.
- The above means that correct setup using isolate-user-vlan is that the upstream interface shall have the promiscious (isolate-user-vlan) vlan as pvid, while downstream interfaces shall have the host (secondary) vlan as pvid.
However I have still not found out any good debug command I can use to verify that the isolate-user-vlan is properly setup and functioning as it supposed to (other than performing a physical test involving the clients).
Another observation is that the upstream interface doesnt seem to be able to send the promiscious vlan as a tagged vlan (port trunk) - only untagged is supported. Could somebody in here verify this?