Yes the principle behind McCarthy is the UK should not demand an EEA FP for entry
but when there is a RC available. I personally felt the CJEU made it clear that the UK cannot use the Frontier Protocol to justify not accepting RCs for entry:
64 It follows that Article 1 of Protocol No 20 authorises the United Kingdom to verify whether a person seeking to enter its territory in fact fulfils the conditions for entry, including those provided for by EU law. On the other hand, it does not permit the United Kingdom to determine the conditions for entry of persons who have a right of entry under EU law and, in particular, to impose upon them extra conditions for entry or conditions other than those provided for by EU law.
65 This is precisely the case here. By requiring an EEA family permit to be obtained in advance, the national legislation at issue in the main proceedings prescribes, for family members of a Union citizen who are not nationals of a Member State and who are in possession of a valid residence card issued under Article 10 of Directive 2004/38, a condition for entry which is additional to the conditions for entry provided for in Article 5 of the directive, and not simply verification of those conditions ‘at frontiers’.
From my interpretation of the above, RC holders are in fact "persons who have a right of entry under EU law", because that phrase refers to RC holders in paragraph 65. So, I take from that to mean that RC holders establish a right of entry just by holding a RC. Plus from my reading of the above, the UK cannot justify using the Frontier Protocol to impose extra conditions on the holder's entry outside of the remit of Article 5, although they can use the Protocol to check whether there is a genuine RC and whether the holder and his EEA relative are actually present which is what I understood by "verification of those conditions [in Article 5]". So it follows from that, to me anyway, that the UK cannot force a RC holder to undergo any further checks like proving that they are in a durable relationship, especially as mentioned by acme4242 a RC in effect already proves they have that relationship.
Plus I concur with 357mag, the residence card is in principle more than in effect a visa; it also signifies that the family is already resident within the EEA. Thus in the spirit of free movement, it seems wrong that a family member with RC cannot travel freely with his EEA relative throughout the EEA territory. Recital 8 of the Preamble of the Directive seems to support this idea:
(8) With a view to facilitating the free movement of family members who are not nationals of a Member State, those who have already obtained a residence card should be exempted from the requirement to obtain an entry visa ...