Hi satyr and welcome. I've taken Roaccutane, got good results from it athough I felt dead on my feet after 4 months, felt like my skin was going to fall off it was so dry, so hello to nosebleeds in month 4 too

.. hehehe. Anyway, have you read the leaflet contained within the box, because I have it right here, and maybe you 'missed' it just like I 'accidently' 'missed' it but it states no strenuous physical activity whilst on the course of medication... I can tell you, however that it is perfectly safe to take protein whilst taking accutane, PERFECTLY safe, you need to take it directly after a workout whilst getting it throughout the day from food.
Generally you need 1g protein per LEAN lb of
body mass, you don't want to feed the fat here. And also, I can tell you that I was taking protoplex which has a lot of vitamin A in it .. I mixed the stuff with milk, for the whole 4 months I was on roc, and I was having checkups every month and she didn't notice anything, well except the stretch marks

.... all levels were ok....when it says do not consume excessive vitamin A, it means just that... don't consume large amounts of it, not 'don't consume any' . Remember doctors will always tell you the super safe way no matter how badly it might temporarily change your lifestyle just so they can cover themselves that you will be alright.
Finally ... where did you get that creatine is liver toxic ? It's not and you can take it whilst on roc, however you will want to watch your hydration levels extremely closely if your going to take it.
For the record I'm not a doctor, these are my opinions, which I consider and trust to be true.