Thanks, everyone, for your contributions. Didn't know that what I did would result in some many responses...
I thought I was very clear in my drawings and descriptions, so not sure what is unclear still. Happy to clarify, though.
Anyway, let's forget for a minute that my extension socket is NTE5. I can clearly see some differences: better connection speed using nte2005 and fewer errors, in comparison to openreach's 5c. The GEA test isn't showing anything wrong with the wiring or socket by the way etc. I explained the reason the bell wire is there, too.
However, I do take onboard the mention of another resistor and capacitor on my extension socket. So, the questions are:
a) are these not going to make further improvements to the connection quality, especially that ADSLNation advertised their products as having more capacitors/resistors than any other filter?
b) how does having another resistor/capacitor on the extension socket (remember, let's not worry too much about the names here) compare to the filters embedded in the Zyxel 8924 then?
c) how can an extra capacitor, whose function is to protect from extra voltage spikes, be seen as 'confusing the circuit'?
Happy to remove the extra capacitor and resistor if needs be, or would be grateful if anyone would point me to a socket that is NTE5 like so that it can accept the ADSLNation plate.