You would have probably noticed the answer from the Home Office is saying what we have already established, that
Section 3C does NOT protect EEA applications, in other words, that the applicant's current leave is not extended:
I can confirm that an application for an EEA Residence card is not covered under Section 3C.
Also, that confirmation of rights does not need an application for leave to remain:
An application made under the (European Economic Area) Regulations 2006 (“the regulations”) seeks confirmation that rights under the regulations are being exercised and, as such, does not require and applicant to hold leave to enter or remain.
But here is the thing: the above only applies to direct family members. As an extended family member you do NOT have automatic rights; you have those rights only when your card is issued (this is in line with vinny's opinion, if you get issued the EEA card you are good, if not, you are not).
In short, since an application of this nature is currently with us, you would not be classed as an overstayer when your previous leave expires and we would not expect you to leave the UK.
In my opinion, the caseworker who answered does not seem to know what they are doing...
"since an application of this nature is currently with us," is not sufficient condition to conclude the applicant does not need leave to remain. They are treating the applicant as if they were a direct family member, which they are NOT. Their conclusion does NOT follow from the previous two statements.
This is as far as I can go in terms of interpretations here, I'll let more senior and knowledgeable members to confirm, correct me, or comment further.
My posts express what I believe are the facts, based on the best of my knowledge, about the topics discussed in this forum. They do not constitute immigration advice.
you’re kind to inform of your situation to help others.