Taking your points in order
JDubya said:
...Given that the van will not be connected to the mains and will only be connected to a 12v leisure battery - does "in the context mains electric and gas matters [static caravans] are legally treated in the same way as domestic dwellings and alterations to installations must conform to the relevant regulations." apply?
I cannot tell you if the regulations are relaxed if the dwelling is not connected to a mains electrical supply. you should seek professional advice on that point.
But I can tell you that the gas system is treated as its its a domestic dwelling as the regulation specifically exclude touring caravans and self propelled motor homes owned by the user. As your is neither a touring caravan or a motorhome the gas side definitely does come under the regulations, and you should be made aware it is a criminal offence to work on a gas system unless you are competent and currently certified to do so.
JDubya said:
...
I understand that the cable colours could be confusing but I plan on disconnecting/removing all electrical outlets and simply use the existing wires for replacing the current light fixtures with 12v LED lights and as there is no mains power within hundreds of feet of the van - there will be no opportunity for anyone to 'wire mains to 12V equipment'.
Bearing in mind the uncertainty if teh regulations apply to the wiring, I still advise against using the same wiring becasue if you sell the caravan on the non conventional wiring could be an issue.
JDubya said:
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Would the gauge of the 230V wires work safely and effectively?
Having made the previous point, it's irrelevant if the gauge of the 230V wiring would be adequate or not, I simply do not recommend it.
Whether you follow my recommendations is up to you, but I have given you the benefit of my professional experience and point of view.