Hi! I'm no expert but since I started Insanity, (last week Monday.) I've lost 5 pounds, and I'm not following their meal plan to a T, although I do use a lot of the recipes, and also make up recipes based on what they recommend. Their meal plan is more so for you to have a balance between good fats/proteins/carbs. Calorie counting alone isn't going to cut it for Insanity, I think. If you on your own are able to maintain the correct ratio of fats/carbs/proteins, then go for it, but what I would do is try the plan for a few days so you get a hang of how they balance them.
As for stopping and muscle loss etc, again I'm no expert, but I think if you're starting Insanity you should be starting it as part of your lifestyle change ie. a lifstyle that involves a good bit of activity, so I think you can mentain the weight loss if you continue a workout plan. Most people do Insanity more than once, or move on to Asylum or do the P90X/Insanity Hybrid. But just stay active and I doubt you'll have to worry about muscle loss (unless you starve yourself, I suppose)
ETA: Are you starting Insanity out of nowhere or are you coming off of another fitness video?? I ask because Insanity is not for beginners!
I would have been really discouraged if I started this as my first workout regimen because it is really challenging.