Welcome! I will do my best to answer some of your questions, and I'm sure the other folks in here will chime in as well. As a side note, I'm quite familiar with the P-90 structure, as I have completed the P90-X and P90-X2 programs.
1) It's very understandable that you are hesitant to eat more. There are two ways you can go about this. One, up your calories by 400-500 for a couple of weeks to a month, and rigidly track your progress to see how much you gain. You can then adjust accordingly. Two, you can start with upping your calories by 100-200 for a couple of weeks, then every other week bump them up another 100, tracking your progress all the while to determine your sweet spot. Don't be afraid to increase; you won't put on wait very quickly unless you're eating quite a bit above your TDEE (like 700 calories or more).
2) I would heartily recommend graduating to some heavier weights with lower reps. If your goal is strength and size, you need to be topping out at no more than 10 reps or so on any exercise. Personally, I have a four-week cycle. For the first week I aim to fail within 8-10 reps. From there, it goes to 6-7, 4-5, and then 2-3 is the final week (meaning if I can lift the weight more than three times, it is too light). The kind of training you've been doing is great for endurance, but your muscle growth and strength will be more limited than they will with lower rep ranges.
3) Caloric deficit is not all that matters, but it is the most important factor. Fat is critical for hormone production (yes, saturated fat too!), which will keep you going strong for the long run. Carbs vary more from person to person, but are usually important for energy and performance, and can also have a lasting effect on your hormone production if you are carb-deficient for a long enough time. Ectogemia had some great posts about this subject further back in the thread, if you can find them and want to learn a bit more.
4) You're correct; don't worry too much about sodium in the long run for fitness, unless it's related to health issues. The only time I worry about sodium is when getting into a temporary "best-shape" physique, for photos and competitions, etc.
5) Your break needs depend upon how hard you're pushing yourself. If you look at your workouts and can objectively say you're pushing yourself to your limits 4-5 times a week, it's probably best to take a deload week once every 1-2 months. (By limits, I would define this as lifting to failure 4-6 (or more) times per workout, while working out for 45 minutes or more). This is a bit of a grey area though, as it varies from person to person and really requires an objective self-evaluation. You'll also have weeks where your body just feels fatigued and ripe for injury, and that's another sign. By not taking these deload (or rest) weeks, you're also frying your Central Nervous System, which can lead to nastier results down the road than just strained muscles.
6) Whether or not it's effective depends on what you're trying to achieve. Doing heavier weights for lower reps is going in a fairly different direction from higher rep training with HIIT. What exactly are your short and long-term goals? We'll be able to help you a bit more once we know those.