As you have discovered, adding more splines to try to get the right shape is usually a losing game. You usually want to have the minimum number of curves that you can get away with to get the smoothest surfaces. You have also discovered that sometimes it is better to split your surfaces up into a number of areas that have different characters rather than try to make one surface form the whole thing. Just as you had to split the top portion up into three separate surfaces, you may need to split the bottom surface up into several surfaces. It all depends on the shape you want.
You also may run into some areas where the boundary surface just doesn't seem to do what you want. You might want to try a Fill surface in such areas.
I'm not sure how few surface features you're using for this shape but I don't think you'll ever get it satisfactory by doing the entire shape with one boundary surface, I might suggest stepping back a bit and strategizing your approach. If I were doing that shape I would recognize that the large lower area is defines the over all design form and that the handle area is secondary and attached to the main form. If you break it down that way in creating your surfaces in SW, I think you might be more successful. Perhaps Sweep or Boundary the main form, then sweep your handle form, trim material back between the two and add a boundary surf to blend between them.