yup its a gas but can get it in a spray can format..like deodrantsco2 is a gas so i am not sure what you mean by a spray. anyway, co2 addition makes some difference if you have everything else optimized. usually, you'll get more bang for your buck from adding more light.
nope, that won't work at all. the co2 is what the plants breathe in and must be supplied throughout the day for it to have any noticeable effect. people who are serious about co2 enhancement use either large tanks of gas or co2 generators that provide constant co2.