Vlan – What VLAN to set as trunk’s native VLAN

Hope I understand what you are asking here (your writing is long and my memory is short :))

The HP untagged VLAN on a tagged interface functions similarly as the native VLAN you configure on Cisco switch trunk interface (actually tag/untag makes more sense than access/trunk technically speaking), the VLAN 8 in your case.

On your router interface facing HP switch, it is running as access/untagged port. So all ingress traffic on that router interface is VLAN 69; meantime router will remove VLAN 69 tag of the egree traffic to switch and when traffic arrives on switch, it will be VLAN 8. However if you have traffic marked with other VLAN, like VLAN 10 and send the traffic to router as the interface facing router is tagged/trunk port, router will not be able to understand it as the edge interface is expecting something without VLAN tag.

There is no native VLAN mismatch just like you connect a laptop to a trunk interface with certain native VLAN configured and switch does not complain about mismatch, right?

Hope this helps.

CDP (LLDP) lists how the each port is configured. Cisco, in their infinite wisdom, thinks access ports in different VLANs are an error. They aren't, but it could be a misconfiguration. Disable CDP and the switch will shut up about it.