Janet- I don't drink diet sodas but I do drink coffee black but most of the time I drink it with either almond, coconut, and lately with cashew milk. The SBD does allow both, I believe the only 2 cups of coffee and you can use artificial sweeteners but I don't. The diet soda (pop) depends on how it affects you. If you biggest problem is sweets then the diet pop could give you lots of cravings.
As far as the SBD being mostly plant base, the answer is no. SBD promotes low fat, moderate good carbs and moderate protein. You can make it plant base, but make sure you are getting your protein from other sources.