Is my disc brake caliper correctly mounted on the frame?
The frame design dictates caliper placement. In your case the mount is between the seat and chain stays. You are correct that the more common method is to run the brake cable along the top tube and down the seat stay. This allows for fewer and less sharp bends in the cable. Running the cable along the toptube and down the seatpost to the chainstay will incorporate two severe bends in the cable. An alternative to this is to run the cable along the front down tube over the bottom bracket along the chain stay. The cable length will be longer than the original but it will minimize the number of bends. You will most likely have to purchase some additional cable stops/brackets for the downtube and the chainstay