There does seem to be a general agreement that a settee is fancier and less comfortable, which, yes, makes sense. I like your graded distinctions, though I think that for me, a couch is bigger than a sofa.

I was going to suggest your 'chaise lounge' is a typo for 'chaise longue', but corvidology above cited 'lounge' as an alternate for couch, so perhaps a chaise lounge is what it is now called. I can imagine that the language has slipped it from one name to another that makes more sense for people who don't speak a lot of French.