I think you probably could do Dukan if serious, but it will be tricky and take longer. But what's a week or two when it's your health on the line?! The dairy stuff is good for filling up and giving satisfaction quickly. If you can't do dairy, will need to fill gaps in other ways.
Are you sure IBS is due to lactose intolerance? Maybe the daily oatbran will help settle things down?
I think soya substitute is ok on pv days, but wouldn't like to say ok for pp. But if that's the only way you can do it, then why not try for a few weeks and see?