Legally, the situation looks bleak until you receive your RC. For the bulk of this post I will assume that the relevant position of extended family members is determined by Regulation
7(3) of the EEA Regulations:
Subject to paragraph (4), a person who is an extended family member and has been issued with an EEA family permit, a registration certificate or a residence card shall be treated as the family member of the relevant EEA national for as long as he continues to satisfy the conditions in regulation 8(2), (3), (4) or (5) in relation to that EEA national and the permit, certificate or card has not ceased to be valid or been revoked.
Your problem is that the FP has expired and you have not yet been issued with a residence card - or a new family permit.
Now there are two pieces of law that may contradict the paragraph I quoted. Firstly, there is a widespread belief, possibly ill-founded, that the principles of Section
3C of the Immigration Act 1971 apply to extended family members, and so anything resembling a permission should continue while you are in the UK waiting for the decision of an application. If your CoA confirmed your right to work, that would support the idea. Note, however, that such a continuation ceases if one leaves the UK, so even Section 3C is of no assistance for the return from your trip to the US.
Secondly, there is Article 25(1) of Directive 2004/38/EC:
Possession of a registration certificate as referred to in Article 8, of a document certifying permanent residence, of a certificate attesting submission of an application for a family member residence card, of a residence card or of a permanent residence card, may under no circumstances be made a precondition for the exercise of a right or the completion of an administrative formality, as entitlement to rights may be attested by any other means of proof.
This appears to contradict EEA Regulation 7(3), which says you no longer have the right to be treated as a family member. On the other hand, it might be arguable that your claim to be a durable partner cannot be assessed by anyone but an agent of the government, so the premiss of that part of the article is wrong.
If Regulation 7(3) is indeed the law (case law may have overridden it), then a number of problems arise:
(1) My reading of the Immigration Act 2016 says it is illegal for your to work until you get your RC. This is based on the assumption that someone who has no right to stay in the UK under the EEA Regulations, but entered under them, 'needs leave to remain' in the legal sense of the words. This law has no explicit exemption for EEA nationals, who it seems in some cases might breach it by working but not enough to thereby be a 'worker' and therefore legal. The CoA may have given your employer a statutory excuse, but you are breaking the law and your earnings since the card expired (or 12 July, if later) may be confiscated as the 'proceeds of crime'. He is also criminally liable if he knows you are an extended family member.
(2) You cannot turn up at the British border (e.g. at Heathrow) and demand admittance as the family member of an EEA national who is a qualified person. You have no right to be treated as a family member. (There might be an obscure loophole that allows you to apply for a family permit at the airport, but I have no confidence that it exists.)
At present, you only have a right to reside in the UK if you are the (extended) family member of a qualified person. As this has not yet been fully conceded by the Home Office, they may retain your US passport until an RC is issued - or they decide to remove you from the UK. The US consulate in the UK are aware that the Home Office is aware, and we have heard that they will issue a second US passport in circumstances like this. I believe this last problem looms whatever the law on your status actually is.