onto the sofa, rejected, even after having been "brain washed" to use onto, even when it sounded awkward. Yes, it may sound awkward here, but I would argue that "ra" emphasize the direction and justifies the use of onto rather than on.

"The girl enters and sits down on the couch" is pretty correct I think.

"Couch" was rejected. But a Couch is the same as a sofa.

couch and sofa are synonyms.

Why is it kanapéra and not kanapére?