Hi Dr. Ake,
I’m sorry to hear that your Nest Alexander Signature Soft isn’t meeting with your needs. 
At least it does have a good return policy and your were wise enough to choose something that yo could try and return if it didn’t turn out as well as you had hoped. You are correct that it is a quite plush product, with the 1.5’ quilt polyfoam and then the total of 4" of memory foam, and this much plush material certainly will allow for you to sleep a bit more “in” the mattress. I’m not sure of the focus of your comment regarding the 13" being too thick – perhaps you meant for your specific application?
You can read a bit about the construction of the Luxi here. The complete specifications are listed in the Simplified Choice Thread here.
Engineered polyfoam designs similar to this are more common in Europe and I would not have the concern of the columns falling apart after only a few years.
I can’t comment as I am unaware of the methods that Luxi uses to promote themselves, but an internet search pulls up quite a bit of information about them, mostly “reviews” from referral sites, which is common with many online mattress companies. I wouldn’t use the fact of whether or not it is mentioned frequently online as one of my main points for considering the product.
A forum search on Luxi (just click on the link) will bring up quite a bit more information about them on the site as well. They do use better quality materials and they have a unique design from someone very knowledgeable in the foam industry, so they are certainly are worth considering. I did question the validity of the one post you are referencing here on the site, and I let it stand more as an example of someone with a personal vendetta unsuccessfully attempting to create negative feedback about a product, which actually ends up having the opposite effect and reflects more negatively on the poster.
That is a solution that is more common than many people realize! 
Phoenix