Question 1) Are my calories set at 1800 correct?
Setting calories are estimates, so who knows. You have to try it out and see for yourself. No one can give you a solid answer on that.
Question 2) How would my macros change?
I am not sure if I am understanding your question. Why would you macros change? Just do your regular diet plan and incorporate some free meals and refeeds (as specified in the book).
Question 3) I need a minimum of 100 grams of carbs per day when dieting in a deficit, correct?
This is also kind of a weird/vague question. Why would you need 100g of carbs a day? It has been a while since I read the book and I do not recall any specific dietary recommendations such as eating 100g of carbs a day.
You did mention a low carb diet. If that is what you prefer to do. I prefer low carb as well. I believe that macros are highly overrated. No one eats one specific macronutrient. We all eat carbs,proteins, and fats. In only a few situations I recommend keeping track of macros.
If you are a competitive athlete, if you are hungry on your meal plan. If you are hungry on your meal plan you should pay attention to your fiber and protein intake. To do that you would have to count macros. You should also count macros if you are if you have medical issues or are doing a highly restrictive diet such as RFL or possibly if you have medical issues that can get better eating specific foods(macros).
Most people do not fall into those categories, I really believe that most people do not need to count their macros. Counting macros add more complexity to a meal plan and this would increase your chance of failure.
I think you're asking the question on how to calculate your macros for low carb.? If you are, then figure out your ideal weight(look online) eat roughly .80-1.0g of protein per pound of body weight. You said you want to eat 100g of carbs? If so add 100g of carbs to that and fill the rest of your calories with fat.
My ideal weight is around 200lbs.
Metabolic rate(when not active) = 2,700 calories.
protein = 200g * 0.8 = 160g protein = 640 calories
100g of carbs = 100*4 = 400 calories
calories left for fat = 2,700 - (400 + 640) = 1660 calories of fat
fat grams = 1660/9 = 184g
I made a minor mistake. I calculated my macros for maintenance. If you're going to do these calculations use how many calories you plan to eat. In your case, you said it was 1,800 calories. Use that number where I put in "Metabolic rate".