The joinery you show isn't terribly unusual. Sam Maloof would approve. I agree withRich that Artisan or SubD would be good options. I would be inclined to model just one third of it and without the joinery initially. I would make the third a solid object (group or component) and then model the leg joinery to use as a cutter for use with the Solid Tools or better, Bool Tools 2 to make the seat-side of the joint.
@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.