I would make it in two parts. The bottom part is an extruded rounded rectangle with a boolean operation with a sphere. For the different forms of the top part you can use different cross-section and add a spiral. And then marry the parts as one bottle. In short, Boolean operations, based on spline modelled parts would be my first way to achieve this. Maybe the rounded upper part of the lower half (are you still with me) can me created with a 'bulge' modifier. In that case you could keep the rounded bevel from the cross-section in the Spline Modeller. But then you still had to make it in two parts. It is a hard one to tackle. (I think this is really easy in Amapi, but i can't get myself to stare in baffled wonder at that program anymore...) If I have some time I am going to try something myself, just for the sake of it...