I fixed the problem of the squishy foam used by the manufacturer.  I took the foam from one of the cushion bottoms down to the local foam store.  I asked for and got the best foam they had, and had them cut two to fix the cover.
Made such a huge difference.  Unfortunately, the sofa foam is another story.  It's a much more complicated foam cut, and haven't tackled it yet.
Back to the dinette for a moment.  I left the original foam in the back rest cushions.  They are fine with no more pressure than they get.