Thought shapes with curves is always more difficult to cut? Anyhow, this is just a small issue, doubt there'll be much difference in price when cutting square or round. Even if cut round, u can fit in square downlights & vice versa.
About cover, some of the downlight cover is provided because it cut down the glare if we stare it directly, ie when we lay down. Notice some cover have been blurred out? My observation is cover type is suitable for residential while the opposite is for commercial.