Clarification on the calorie/BMR/TDEE numbers game?

Sindaelys

I found the brilliant "In Place of a Road Map" (by Helloitsdan) thread today. It was clear, concise, and perfect for the newb that I am. I follow the directions, do my measurements, plug numbers, and then... WOAH! I'm eating 600 calories fewer than I should be? AND below BMR?

It makes sense to me that I should at least be eating my BMR calories; I'm not in a coma, after all. Output would still be greater than input. This leads to a couple of questions:

Why does myfitnesspal recommend eating under that limit?
Why does their BMR calculator show me 100 calories under the average of different ones?
Is theirs really "more acurate"?

So I move on from BMR. If I'm not in a coma, I need to figure out my TDEE. Since I don't know how much I walk around yet, I'm stuck using the "sedentary/lightly active/moderately active" etc. presets. So:

Is this how active am I during the week, or how active am I during a day?
Sedentary vs. lightly active. Mostly I sit around all day, but I do walk around town a bit 5 days a week. Does this add up to "lightly active"?

Phew! I think that's all my questions! Also, if anyone has advice on macro breakdown or how to hit my vitamin/mineral goals, I'll take that too!