The report seems to agree with you "The RAIB investigation has found that instructions given to car drivers using this, and similar, level crossings were inadequate. It also found that Network Rail’s method for ensuring that vehicle drivers have an adequate view of approaching trains was incompatible with the characteristics of both the car involved in the accident and many of the vehicles expected to use crossings of this type."
It does seem unreasonable to expect the landowners to escort users across, although if I have understood it correctly they aimed to do this on their own initiative. The report gives me the impression that for once they, including the motorist involved, were more aware of the risk than the railway were.
The official definition of large/slow moving includes "less than 5mph". I'm not sure that I would accept that a car with a trailer is necessarily going to do 5 mph in a location like this; I suspect a driver with a cautious nature or lacking in experience with trailers might well drive unduly slowly over a crossing, as he might over a speed bump. Given the outward opening gates and very poor visibility, a cautious driver appreciating the risk would return to his car, then draw forward slowly to a point where he can see further along the railway. He might well misjudge the fouling point, but when he saw an approaching train his trailer could then cause him difficulty reversing clear. Whilst probably not the case with the driver involved, inexperience with trailers is likely to be more of a risk at leisure spots such as boating or caravan/camping venues. It might even be safer with a "boy racer" - at least you're not on the crossing for as long!
On the other hand tractor drivers are generally much higher and can often see over obstacles such as hedges.
I am intrigued by the mention of this experimental "Wave" system to control red lights (unfortunately not in use at the time) which I have not heard of before. What advantage is anticipated over a treadle or track circuit - reliability, less vulnerability to damage, cost? It is sound operated To be effective, it has to detect the train without giving false warning because of wind noise, cattle lowing etc. How is this achieved - some sort of intelligent analysis of sound frequencies? By the noise of approaching trains? Perhaps by the train whistle, relying on the driver to obey the whistle board, which he did not do on this occasion. This system is not considered as being part of the signalling system - why not, when wig-wags and miniature warning lights normally are? Is it fail-safe?