Wow! That was easy (because you know what you're doing). Thank you so much!

It may be your mouse. I have had this, too. When you double click with the middle mouse button, it is actually a sort of mouse shortcut to quickly zoom into (center the view at) what you are double clicking. Now when my mouse got broken, it sometimes double clicked automatically which drove me crazy. If you double click into empty space (even "seemingly" near your component), you may be taken far away. Check if another mouse does the same.

Ah YES!, gregw, that did it! Including the top edge of the extrusion profile closed the final miter. Many thanks Gaieus, Alan, and gregw for your help. I appreciate it greatly. Whew! Now I can move on to other things. Thank you.

You know you can get the face back by tracing over one or more edges, just be sure that the endpoint inferences show when you hover with the pencil tool.