I wish I had a couple pieces on hand, but alas, not yet. Can C-Beam fit into it's own channel? See attached jpg.
Do you mean "can a 4020 fit into a c beam"? If so, yes, it should but could depend on who the manufacturer is.
Theoretically yes. In practice it may take a bit of sanding or a tap with a mallet. It all comes down to the tolerances in the extrusion.
I can verify that the Cbeams purchased from OpenBuilds that are in my shop do indeed fit beautifully.