I was a hardline on my first 30 week run and people around me were drinking flavoured waters with malic acid and skinny lattes (I hate milk so I never really bothered with that anyway) I never even had Marigold bullion - I spent a fortune on the official Lighterlife sachets (and I used to have the full allowance of that and water flavourings every day without fail - I was spending an extra 15 quid a week on thin air, practically!) Doing it hardline worked for me first time round and I would recommend doing it that way for anyone who likes playing by the rules or has a tendency to bend the rules as it keeps you in check.
Now - I don't like telling people they can bend the rules precisely because of that first time round I had, but I have two failed restarts under my belt so I wanted to get through this (The first 8 weeks are always the toughest for me) So - this being my 3rd restart in as many years and the beginning gets tougher every time...so yes - after about day 5 once I was well and truly 'doing it' I had Coke Zero (I know for a fact I am pretty hard to shake out of ketosis) and it helped me keep my mind on the game. Since I passed week 10 though, I forget I even have it in the fridge some days, I don't have water flavourings, I think I've had about 5 cups of bullion in the entire 90 days I've been back on plan...and next Thurs we'll see how close to my first time 100 day weight loss I am - anything around 4st would imply that Coke zero has had no effect on my losses.
I would say, try doing it by the book. If it gets tough, keep trying it by the book. You may surprise yourself.
The other other reason? The water flavourings give me a runaway bum. They are why I had to come off it last time. This time round I have not had one spoonful and have had a normal (for CD at anyrate) properly functioning bottom.
But you want my honest opinion? Those optional extras are there to up the profit margins and I couldn't really afford them back then and I can't afford them now, so I figure a couple of quid a week on Coke Zero and a tub of Marigold Bullion versus having to come off CD because I've blown my monthly food budget...well - there's no contest, is there?