I haven't seen the MMR plates in person but I'd suspect the need for the modified nut on the Vorschlag plates has something to do with the geometry of the plate itself.Vorschlag uses different custom made longer nut that drops in around the shaft. I reused the OEM nylon nut.
I had mine installed last week by a shop here. He noted the same problem on the passenger side top nut. Interestingly enough he said that was the exact nut he couldn't get to tighten down on MM plates for a different generation Mustang and is having me keep an eye on it. After a track day with it in place I haven't noticed any backing off of the nut and we were able to torque it to 30 ft-lbs I believe but it will continue to spin if you put more torque on it.I did try it with impact this morning and keeps on spinning. I will contact mm and see what they say.
Hold the shock shaft with lock pliers wrapped with thick cloth or use rubber lined lock pliers if you have them.