I've had an fissure in the past, before I got pregnant and had DS - it was awful, you have my sympathy! It got pretty bad before I even knew what it was, and took ages to heal - it was basically there for 2 years and I have had several others, much smaller, since then so I guess I am prone to them...
FWIW - you are very unlikely to be offered a CS to avoid another fissure, unless you happen to get a very sympathetic consultant. It just isn't seen as a bad enough complication. Not saying I agree with that, at all' but that's how it is.
i had a planned CS for my 1st DC and that meant I escaped any perineal trauma and I was very grateful I didn't get a worse/new fissure. (I did have to warn the nurse who was giving me a painkiller suppository to go easy, and she was very sympathetic, luckily).
I didn't have any constipation post CS, but then I ate sensibly with plenty of fruit and stayed well hydrated. I didn't actually get my initial fissure via constipation, btw, I got it through the opposite, diarrhea after food poisoning. So over doing the laxatives can make the problem worse, not solve it. what fissure sufferers need is stool softeners, not laxatives - there's a difference.
So really, unless you are adamant that you want a CS and are very determined to fight your corner, you are very unlikely to be offered one purely on the grounds of a fissure. personally I think you are much MUCH likely to get one if you have a planned CS than an instrumental VB, but of course there are many other factors to take into consideration. the gp I'ce seen about my fissures said that it was damn lucky I had had a CS not a vb, but that was just in respect of not getting any more fissures.
Good luck - and if you haven't tried it, Xyloproct is a wonderful bum anaesthetic! Need it on prescription though.