@Aartrace1 I think you're getting too caught up in the split tool and making things perfect. Think about what your intent is with this Layout sheet with the site, building outline and contours. Now think about all the time and effort you've invested in just trying to cut these contour lines and issues with the viewports. I think in the overall scheme of things the contours are just background information to your overall drawing. Besides, if you really want to be accurate, aside from the existing contour lines you would also need to show the new contour lines because there would be re-grading to allow for the building footprint, retaining walls, paved areas, site drainage, etc. As I like to tell my staff - work hard but work smart.

@Aartrace1
If your contours and building outline are separate references as you initially stated then why not create a closed polyline with a white fill that is layered (sandwiched) between the contour reference and the building outline reference -- this to mask the contour lines below. If these items are all on the same layer then use the arrange to bring each object (front/back) in the correct sequence. Or you could place each on separate layers to achieve the same with the correct hierarchy. By just masking the contours you won't have to split each line.