Hello Colin (Yorkshire),
Sadly I have to take issue with your statement that 'rules don't make sense or conflict with other rules' and the inference that it is not necessary to get bogged down in the technicalities.
The rules are actually logical if you read them, but they are complex, and you have to be prepared wade through all the sections and riders.
Please note I use the word logical, because there is a logic to them, even though they are complex, which gives them the appearance of not being sensible.
Because they are logical and more importantly legal, you cannot choose to ignore them, you have abide by them if you intend to tow on public road, continue to be covered by your insurance, and to avoid warranty problems with your car.
In their entirety the towing laws cover every situation, as by definition anything that does not comply is illegal and therefore should not used on the roads.
Each current model of car is given its own gross train weight, and they do take into account the different engine options, so what the manufacture says is correct. A good example of this was the MK1 Vauxhall Zafira, where the diesel model had a significantly different GTW to the petrol equivalent.
It has always been the case that you should check the car manufactures max towing limit, as that is the legal measure. You should never assume that 85% is always acceptable. As many others and I have pointed out previously the 85% is not a rule only a guideline that has been suggested by the UK caravan industry. It has no basis in law and is not adopted anywhere else in the EU.
Using internet based outfit matching programmes, cannot be assumed to be accurate, as several flaws in the information they use have been identified - always confirm details for your specific model of car and caravan with the manufactures.
I am not surprised that Caravan dealers do not know the capabilities of all cars, that is not their expertise. Equally Car dealers are not likely to know the limits for every caravan model, however they should know the limits specified for their own products. It would not be a defence in law if a driver uses the argument that a caravan dealer said their combination was legal, when in fact it was not. The law is very clear, that the driver is entirely responsible for the safe and legal operation of a motor vehicle, and ignorance of the relevant laws is not an excuse. So the choice of car and caravan is down to the driver.
In today's society, the relevant law is available from government offices on the Internet, so access to reliable information is available, and therefore it is even less reasonable to claim ignorance.