Hi liveyk
Welcome to the forum.
The first thing to remember that it is not a *requirement* to tow with a WDH. It can certainly be an aide on many vehicles, but on those vehicles where it is recommended not to have one, then you can certainly tow without.
For a factory installed bar, you've seen the comments above. There is a bit of doom and gloom in some of our comments, but at the end of the day, Nissan advise they have fixed the problem with the bar and says it is good enough without a WDH. You can trust that.
Airbags are also mentioned and are a very good way to help the car sit better. These act on different principles and aren't the same as WDHs so have nothing to do with Nissans recommendations. Many have found them beneficial with or without WDHs.
If you definitely want a WDH, it isn't too painless to change the rear bar over to an HR bar, and unlikely that the leasing mob will worry too much if it in the best interest of their investment.
As for the GVM : tow ball weights......well it was my initial post on PCOA that Brad refers to, and one of the reasons I sold the Pathy.
It comes down to how you kit out the car. I kitted mine as an off roader first, and then got the caravan a couple of years after.
Basically, the weight of the vehicle increases through such things as bull bars, winch, heavier tyres, extra fuel tank etc, and also driver and passengers. Once the total weight reaches a certain point, the tow ball weight allowance start reducing on the sliding scale Brad mentions.
So the Pathy Ti (2.5 litre) has a GVM of 2880kg, and a (standard) tare mass of 2212kg. That means 668kg of load before GVM is reached.
80 litres of diesel = 68kg
1 Driver + 1 co-pilot + 1 kiddie = xxx kg (I'll let you answer that) in my case with three kids we were easily hitting 280kg.
So that makes a weight of 2212+68+280= 2560kg. This is 320kg less than GVM, so according to Nissan means somewhere between 250-300kg on the towball. Ok, so that is fine for all models of Expandas. I could even throw a few bags and whatnot in.
However, in my case, I had a lot more weight on the car from bar and winch and wheels and few other bits and pieces, so when I did actually weight it with a full tank, it came in at 2460kg. Add the 280kg of people and it is only 78kg less than GVM (mine was a little different from being an '08 model, but same principle). Thus according to the sliding scale, I'd have a towball allowance of 100-150kg.
So in a long winded answer to your question, you can't 'reduce the GVM' as such. You need to limit the weight going on or in the car, to ensure that the car doesn't reach a weight whereby the towball weight has reduced to be less than the van you want to tow. And really, that means if you don't intend to but a bull bar and bits on, you'll probably be fine with a large proportion of vans - whether a great Expanda or other (lesser) brand
(This isn't just the Pathys either. Nissan is one of the very few with a sliding scale, and similarly apply it to Patrols Xtrails and Dualis also. Toyota on the other hand add their towball weight into their GVM instead so a LC200 with a weight carrying ability of 600kg or whatever is really 450kg if there is 150kg on the towball. The discovery I now have is 350kg on the tow ball until I reach the GVM, and then it immediately drops to 150kg)