What you see as jagged in render isn't actually jagged. A curved surface gives an infinite number of possibilities for light reflection, so computer renditions limit them to a manageable number by fitting facets that approximate the surface, and that's what you see. Workplanes aren't that complex. You've drawn 2D splines, and they have to be on different planes to loft. Each object in your drawing is associated with a coordinate system, and the x,y plane of that system is a workplane. Select even a 3D curve and you see the origin and axes that it's associated with, transformations to the object are relative to those. TC lets you put the origin anywhere, and orient its axes however you wish, so any translation and transformation of any object is possible relative to any point with any orientation. That's quite a simple concept, yet almost infinitely flexible - when you understand how to use it with TC.
"Won't shell" refers to one of TC's capabilities. When you loft open shapes, you create a zero-thickness surface. When you loft closed figures, you create enclosed volumes. You can "shell" either, which gives thickness to a surface, and makes volume solids hollow with constant-thickness walls. That can't happen in all instances. If a surface is too irregular or falls outside some other criteria, shelling fails. Lofting is a pressure, or smoothness, distribution system, and lofting an open surface is less demanding than lofting a closed one. The ability to shell is a good indicator of whether or not a surface is smooth. You've oriented your fin flat on the world workplane, I've altered the orientation of the tip spline, so the resulting surface dips into the space below z=0, then sliced off the part that does so. TC, like other CAD applications, adds and subtracts 3D objects from one another. Slicing is a boolean operation that separates two parts of an object with a plane.
"Coincident splines" just means two or more splines occupying the same space.
TC is capable of making your fins in many more ways than one, but its capability has a learning curve. It can't create the objects you want it to unless you know how to tell it to.