You are right in that you should try to spread your calories out evenly throughout the day, preferably in 5-6 small meals. More importantly, though, is the amount of calories you burn/consume -- too few calories over a period of time will slow your metabolism which in turn will affect weight loss; too many calories and you will gain weight. You don't mention what type of exercises you're doing, but any type of aerobic exercise will burn a lot of calories, and when that weight comes off, a portion of it will invariably be muscle mass. Since muscles help burn calories and determine metabolic rate, lifting weights and proper nutrition will build muscle and minimize muscle loss, thereby preventing decreases in metabolism.
As for how much your metabolism will rise after exercise, it's hard to say -- it could be 5% or 10% above baseline for a few hours or for the rest of the day. Depends how hard you work. Eating a mixture of carbs and protein after a workout can also help keep your metabolism elevated, aid in protein synthesis, and replenish glycogen stores. Since building a pound of muscle can take time, significant changes in body composition and long-term changes in metabolic rate will likely take longer than 20 days to become noticeable.
Thermogenic aids (eg. herbal supplements, substances with caffeine) probably won't raise your metabolism much to any significant degree. Also, various compound interactions with THC metabolites may or may not have an undesirable effect on THC elimination rates.
Regarding your duration of marijuana usage, THC metabolites in heavy users (more than 1 joint a day) apparently can be detected in the urine, in levels greater than the legal limit, for up to 6 weeks after your last use; THC half-life of as long as 10 days. For non-heavy users, traces of the drug could appear up to 3 days afterwards; THC half-life of 2-3 days.
For your diet, I would cut out the sugary, empty calorie, no nutrient foods/beverages, such as alcohol and soda, and replace them with some water. If your end-of-the-month test is a urinalysis, having very concentrated urine will not yield good drug concentration readings. To support muscle development and cardio-induced weight loss, your increases in caloric intake (particularly protein) can be met with the addition of fruits, vegetables, legumes, lean protein/cuts of meat, low-fat dairy products, and complex carbs.
Even though I'm not sure how much losing fat mass (either rapidly or slowly) will help your situation since there's no way of knowing the storage distribution of THC in the body (I believe it can also be found in the liver) and you have no control over which fatty tissues you stimulate during exercise, and I am unfamiliar with the mechanisms which regulate THC metabolism or how it is released into the bloodstream... I would say your best bet would probably be to refrain from further marijuana use (including passive inhalation), eat adequately to feed your body/muscles, eat as healthy as possible, don't skimp on the water, and throw in some exercise. Good luck with everything.