I would like to know if a chapter structuring command
    is always mandatory for each node. 

No, it's not mandatory, although it's generally a good idea.

    (table, menu location, updating commands, introducing lists)
    for which the chapter struturing commands are only available
    for Info files.

Yes, some of the short nodes that begin chapters are then followed by an
@heading for the first subsection.  This is even discussed in the manual
itself, I believe.

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