I disagree on your interpretation of Article 8, the Regulation is tacit on re-routing on other airlines. Whilst there is almost certainly no right to compensation, Article 8 is still in play and given there were no alternative flights available to return the passenger home until several months later, re-routing (or reimbursement for travel) on another carrier/mode of transport is certainly in scope of the Regulation.

You are attempting to import a restriction to the Regulation that does not exist.