Your mention of malic acid being okay prompted me to re-check my fave fizz of the moment - Asda Cherry Cola - a Zero-type drink. 'Free From' most nasties, no aspartame, costs around 47p a 2-litre bottle and contains - MALIC ACID!
I am on vegetarian Atkins and to preserve my own ketosis I have to be careful about drinks. I cannot drink endless bottles of water, I get fed up with it, but to keep fluid intake high I often mix the Asda Cherry Cola 50/50 with water. Compromise!
Good luck everyone, you're brave to be doing TFR! xx