Okay, I just found your post about mattress thickness here and it pretty much answered my questions. (There is so much information on this site, it makes my head spin!)
For someone as lightweight as I am, it doesn’t seem like a 14" mattress should be necessary just to get the cushioning and support I need? I am wondering if that is why I found the memory foam mattresses at the stores made me feel like I was sagging at the hips, because I am not heavy enough to sink down to the support layer? I don’t know, this all sure does get complicated. At this point all I know is that 6" of 4lb memory foam on the floor felt better than any of the mattresses I tried so far, lol! (And by ‘felt better’ I mean, my back was blissfully straight and my bones were cushioned). I hope I can find something permanent that gives me the same feel.