...and of course, the mere existence of such a counter-signed key makes that key just as powerful, and valuable, as the root keys (if a piece of malware was somehow signed by that new key, it would just have to carry the signed key-update-request around with it).

If you want the issuance of countersigned keys to be viable in practice, there needs to be provision in the spec for the signed key update request to be limited in scope - say to a particular motherboard serial number and/or date range.