I'm wondering why the basement ducts are located in the centre of each
room and not the outer edges, like the main floor. Is it because
they're in the basement? in the ceiling? was it just simpler/cheaper to
do it this way? It seems to me that placing them in the centre robs the
outer half of the room from some warm air flow since the warm air would
enter the centre of the room and immediately flow towards the return
air ducts in the centre of the house.


It was probably just easier, and I don't think the air-flow patterns
are likely to match what you're expecting. Unless you're getting
fairly high-velocity air flow, coming out a nozzle aimed at the
returns, what should happen is that the warm air comes out of the
supply, and billows out across the ceiling, forming a warm air
layer that slowly lowers as cold air gets sucked out at floor level.