Your argument about the enforcement authority will only get a hearing at the Traffic Penalty Tribunal, as I can't see St ALbans council giving way on that one. Indeed I am not sure if your understanding of the legal aspect here is correct or not, hopefully our adminstrator will give an opinion.
You may well be on much stronger ground with assisted boarding/alighting, so please tell us more about what you did, and how long it took.
Do be aware that councils reject virtually all informal challenges to paper PCNs served at the roadside, as they know that most people (like >95%), then just pay up, as you appear to have done with a previous PCN. Did you know it was a zone for resident permit holders only when you drove in and parked ? The sign is here, and there may be two with one of t hem twisted round: -
https://maps.app.goo.gl/qFHyrsFZVhjfL3Y5AThe traffic order for zones like this normally contains a clause allowing deliveries and boarding/alighting passengers. So I think your best approach is probably to say what you were doing whilst out of the car. Please note, however, that the board/alight activity must only take as long as necessary; stopping to have cup of tea and a chat as well is not permitted so you'd have to explain why the CEO was able to observe your car wthout its driver for six minutes.
