I've tried Atkins but can't do without carbs, it seems. But I also seem to do pretty badly with sugars and starches, so I do still want to avoid them. I don't have a huge amount to lose, but I'm also concerned about slowly gaining over time.

I eat healthy carbs like apples, brown rice, beans, lowfat milk, and oatmeal. I don't always have all of that in the same day, but would it be detrimental to eat them all & on the same day as a serving of peanut butter, coconut oil, and tahini? (I also have protein shakes and egg whites to up my protein intake.)

If I have a high percentage of the day's calories coming from good fat while still eating those carbs, does it really matter, as long as I'm under a certain amount of calories per day? Should I be worried about the percentage of fat in my diet, if the fat is mostly the good kind? (I also have protein shakes and egg whites to up my protein intake.)

I would not worry at all, your plan seems perfect to me. Just make sure you like your own system

I just bought a wonderful book that I would HIGHLY recommend to you...
"Living the Low Carb Life" by Jonny Bowden
It discusses the fat/carb relationship and history as well as tons of other information....
It really helped me alot!
I think-
people vary on how insulin resistant they are, and you probably should start a journal and write down how many carbs you eat with your fat intake & just watch how it effects your weight to find the perfect balance for your body... and if you get a chance to find that book at your local library or bookstore, please check it out!