Tapping control ( or alt on mac) will create a solid but will not solve the problem with the reversed faces.
to create a 'Manifold' Solid you must first get rid of the centerline, then select the face and copy it, Get rid of the original (Erase) then paste -in place the copied Face ,Select,right-click_> reverse faces.
You will then create a manifold, true solid.
It is important to be alert , regarding which side of the face you're 'Pulling' , I always have the Entity Info nearby. It is like putting on a shirt in the morning inside-out , not noticing till someone is telling you.
I presume that, after you created the curves , you'd painted the faces but SketchUp always faces 'down' when making a face with arc's and lines. It is as if you are already looking inside
the box, before pulling the face up to a solid. In doing so, you've painted the 'soon to be ' inside off the solid.
Best workaround is to first make the geometry and later on paint the faces.