I see. What I did wrong is that I shortened one of my subelements to leave a gap, then filled that gap with a new line, and hoped Complex Redefine would work (it didn't: it simply replaced everything north of the gap with the new element), and failing that, I hoped Append Element would work to add this element in the gap.
Now I understand (correctly or not) that if I was going to succeed on the complex redefine, I would have had to *not* modify any elements within the chain, I'd need to make sure the replacement route was bookended by elements that extended to and terminated at the previously defined alignment elements, then construct a new complex element from these alternative elements, and *then* use complex redefine.
I have already worked around the problem I was having by dropping complex and reconstructing, so that ship has sailed, but now I'm wondering if the workflow I pieced together just now would be effectual, and how. I guess I'll try it next time.
And the other video has nothing to tell me how to insert an element to cover the gap between two segments of an existing complex element. It demonstrates only adding something to the end of the chain. I mean, the prompt does say Add/Insert. If I wasn't going to be able to insert it to occupy a gap, I'd understand if the prompt said simply Add. Or, to disambiguate even better: Add To End.