When submitting an extended version, is it mandatory to include all authors of the original paper?
An extended version of a conference paper supersedes the conference paper. A priori, this means that every author of the conference paper is also an author of the journal paper. In same rare cases it could be appropriate to drop an author (if every part they contributed to gets removed), but as a matter of protocol, I would only consider this on request of the relevant author.

This is different from a subsequent paper building on a prior paper from the same project, where a citation to the previous paper suffices. In such a case, one would not copy over important text anyway.