You are correct. An 8000 CCN cannot be used to move goods across a border.
In this instance the Highway carrier will file its conveyance and cargo data. And the Highway carrier will provide its Primary cargo control number to the freight forwarder. The freight forwarder will file its eHBL data for every house bill on the truck load (one or more) referencing the Highway CCN as the Primary CCN in the eHBL transmission – and referencing that Highway CCN as the Previous CCN in the Close Message. The Customs release request will be against the 8000 CCN and it can be a PARS.
Of course, if the Highway carrier pro-bill/ bill of lading is consigned to the importer and its ‘conveyance and cargo data’ provides the shipper/ consignee details, then that is a direct shipment and not covered by eHBL, because there is no house bill contract of carriage. Everything, including PARS releases would be as they are today.