don't try to make it one surface! make the top and bottom surfaces first, then the left annd right side then knit together then surface fill the front and back.
you are trying to much at one time. smaller steps make it easier sometimes.
just my opinion...model maker and prototyper.
Sideswipe-test.SLDPRT.zip 4.7 MB
I didn't open up your part, but you will probably be much better off adding the swipe as another feature. Mixing the smooth countours of the front and back edges with the straight swipe is not going to work out well in the loft.
You may be able to do the swipe as one feature, but to get it to smoothly transition to the original surface, you may end up making the straight swipe with abrupt transitions to the surface and then make a blend surface or surfaces, perhaps with fill or boundary, by cutting the original surface and/or the swipe. I could easily imagine the sequence being to generate a swipe that extends past the body, trimming the swipe and then the body to generate the edges of the fill or boundary surface(s), and then making the transition surface(s). You might end up putting in some 3D splines for guide curves as well.
One of the many great presentations from Solidworks World talked about thinking about surfaces as sides of a box. Even though your shapes are more freeform than solid modeling, making a front then a side then top and bottom (or any other sequence of such) and coming to a closed model, then making it solid. Can help you a great deal.