Yeah, the rear height of the vans does vary. Stow N go vans sit higher in the rear, which may be part of the reason why people say they handle worse than the earlier vans.
You don't need to have the weaker rear leaf springs to have nivomats. My 2004 AWD has the multi-leaf packs, and still has nivomats. The rear is pretty solid, especially since I added half-leaf helper springs. My 2000 T&C was front wheel drive with the monoleafs, and also has nivomats. I had previously added those half-leaf helper springs to that, which helped the rear sag and stiffened it up. Towing a loaded trailer, yeah the rear will bounce a little but it's to be expected. Goes right back to normal after unhooking the trailer. This is both on vans with over 200,000 miles on the original rear suspension.
The non-fitment remarks the vendors use are partly because the nivomats use longer mounting bolts than standard shocks. To convert from standard to nivomat, you have to have the longer bolts. Also, check the rubber bushings in the shock eye mounts of the nivomats. I've read of 2 cases on here where people had to replace them because they were knocking or loose/bouncy.
I had a parts van that I drove home that was very rusty and was AWD with nivomats. It DID bounce, but that was because the rusted away muffler was throwing hot exhaust on the one nivomat until the plastic dust shield melted away and the seal gave out, making it leak. Keeping good exhaust on these vans is very helpful to long life of the vehicle and parts.