First of all up to 2 cups of dairy is allowed in P1 as well as good carbs from some portion of beans and a minimum of 4.5 cups of veggies. So there are really a lot of good carbs available even in P1 if people choose to utilize them. I realize the very first version of the book did not allow milk or yogurt in P1, but it has been in the P1 food list since early 2004. Be sure to check out the semi-current food lists here in the P1 forum and the meal model, both as sticky topics in the P1 forum. They really help put together your own program much more easily than following the menus in the book. The SC book would have pretty much the latest P1 food list, but if you get it you will see the guidelines for dairy and what kinds of milk and yogurt are allowed in P1. (Although the food lists here have that info too. most of the changes since then were mostly cosmetic and removing brand names in place of generic descriptions and that sort of thing!)
Good luck on your continuing SB journey!
P.S. While you might possibly lose 15 lbs in 18 weeks, I certainly wouldn't set that up as a hard and fast expectation. Even if you only lose 5-10 that would be a great success too! This is a longevity and consistency based program that happens when we
change our diet, not go "on" a diet. Those last 5-10 lbs esp. go very slowly and it's often more about patience and just sticking it out than meeting specific timetables. Weight loss tends to happen in its own time and at its own pace for everyone individually. It's great to have goals to plan towards, but don't let yourself get disappointed and discouraged and get off track again if the plan doesn't happen exactly as you would like it too!

Just stay the course and you'll get there eventually and you will be getting healthier all the time too!