Regarding back pain, seat v's waist, its not quite as simple as that.
If you have learned to ride waist harness style, from windsurfing with a straight back hips forward stance, and the power driving down your back leg, this seems not to create to many problems.
This is a little simplisitic, because many folk have differing back problems caused by other aspects (car/bike shunts etc) but as far as pain being created from the activity of kiting itself, I think the waist is safest or the impact harness.
My own experience of back pain, which is muscular and was initially caused by years of abuse offshore waterski racing, before windsurfing and kitesurfing, but is brought about when my back is bent for prolonged periods, or straining with a bent back. And nowadays occurs if I use a low hook position, which of course is the position I most like.
So that brings me to the next point, using the DaKine range as the example, and at the end of the day the brand with most experience of harness building over the years.
With Seat harnesses there were high and low hook positioned harnesses which had dramatically differing effects on those back problems. These functionally worked by either the hook being threaded on the waist belt, or the lower strap that pulled directly under your buttocks. The lower was designed to maximise power through a windsurfing board in slalom racing whilst the higher tended to suit freeriders with less power demand.
Then the XT Seat came along with its combined seat style yet with waist or higher back support and the ability to vary the height of the hook. This evolved into the Fusion Kiting Harness of today. This does give you the ability to moderate both your riding style, and vary the height of the hook and is probably the most suitable harness on the market for those who may not yet have fully developed a varied riding style.
When I posted the details of the Nitrous shorts, I did point out that it might not be ideal for those with lower back problems, and I have to admit although they've turned out to be very popular, my own experience has been that the nagging pain in my back is there right now, after this weekends riding comfortable though the Nitrous shorts might be, and they are wonderful.
But they do have a very low hook position, and they do promote the jolly old toilet stance which I for one find unsettling and in constant fear of my fins breaking out in the bigger puffs, so have to constantly ignore and try and ride as if I'm wearing a waist.
SO in brief, check wether it is the height of the hook that is causing your problem over the riding style of hips thrust forward rather than back in a semi seated style. Then apply whichever harness suits your preferred style, and check a few brands for comfort AND support, sometimes its not just the most comfortable harness which provides adequate support for your back problem, and DO experiment with hook height.