To avoid the whipped cream from deflating or melting, you should avoid mixing hot or warm stuff into it. Firstly, when mixing the melted white chocolate mixture into the whipped cream, make sure it has cooled down completely to room temperature. Ideally, the chocolate should also be chilled but if you chill the chocolate mixture, it will firm up, and that is not the best consistency to mix into the whipped cream either. Hence, suffice if it is cooled to room temperature. Secondly, when mixing the kiwi coulis into the mousse, make sure it is thoroughly chilled. This will help ensure the whipped cream does not melt or deflate.