How much space would they get on the ISS either ways? It'd be a good practice.
For efficiency you'd want to go higher. There'd be a point much higher than 300 km where detethering would put you into a highly elliptical orbit where the periapsis is 300 km. Then you'd just have to burn retrograde to circularize. Off hand I'm not sure what the math for that would be but it should be pretty simple algebra.
An inflatable "house" is conceptually completely possible.
Airplanes have to be small because of air resistance. Space stations have no such constraints.
The mathematical argument of 2D vs 3D still holds.