Hi DaveyJ,
I’m not 10sChick of course and hopefully they will see your post and make some comments but in the meantime I can make some comments as well …
They may have been referring to the thickness of the polyfoam in the Tjome topper (it also has some polyester wadding which would add to the thickness) or they may have just listed the thickness incorrectly. There are also some good sources for many types of toppers (including polyfoam) listed in the component post here if you wish to try something a little thicker or thinner.
Yes … they would have placed it perpendicular to the length of the mattress so it was under the lower back. They probably tucked both sides of the runner under the mattress so the weight of the mattress would secure it in place although they may have used some other method such as the sticky tape you mentioned to secure it as well to keep it from loosening over time.
You’ve probably read this since it was one of the links in a previous reply in this topic but just in case you missed it some of the suggestions in post #4 here may be helpful as well.
Phoenix