Corner models
Draw the corner models with the model’s front facing north like the rest. By default the corner’s extra side (the second backrest of an inner corner, the post of an outer one) is on the right as seen sitting, on the model’s east (+x) side. That’s how Nexo’s and most bought sofa models are drawn. If yours turn the other way, setcorners: left. Lode Items turns the corner model a quarter for the mirrored corner, so one model covers both.
The piece next to a corner treats the corner as part of its row, so it gets no armrest on that side. That works for both kinds of corner: the usual L with the backrests outside, and the other way round with the backrests inside.
Pieces already placed update their shape when something next to them is placed, broken or turned.