Hi Bailey
Here is my opinion

Id stay away from the diet coke, diet sodas in general. The aspartame may cause a stall or spike your sugar cravings. Some of us here have found that aspartame does this to us. If you have licked the diet coke habit, You would probably be best to stay away from it. Use it as a "treat" once in a while maybe? Thats what I do with diet vanilla coke, once every 3-4 weeks (during pms)
As for your OWL, do what works best for you. I increased an average of 5 carbs per day compared to what I was eating, which was usually around 15-18. You might find that if your only eating 15 and you have increased to 25, you may trigger sugar cravings. Then again you might not. It would with me, so just remember that everyone is different and do what is best for you.
Hope this helped