Shimano cog spacing between 7 and 8 is pretty close, about .15mm. Most people can't tell the difference when an 8s shifter is used with a 7s cassette or vice-versa. The upper floating pulley will compensate.
Center pull cantilever brakes with V brake levers? Maybe, if you setup the pads fully extended with the arms spread as wide as possible, and the straddle hanger/crossover cable as low as practical.