Plagiarism Part 3 - Authorship

In times past it was not uncommon for some professors to take credit for their students' work, writings, bright ideas, creativity, or novel solutions. Earning a doctorate was a right of passage in more than the obvious sense; the star pupil earned the right to get proper credit for their efforts. The extent of this practice may be easily exaggerated or perhaps under-counted. I do not hesitate to say that the tide has turned in the last few decades to this practice becoming more generally unacceptable in the academic community.

Yet vestiges of these practices may still thrive. Why? As a hypothetical matter, among a group of highly respected and accomplished scholars is an individual (Z) who frequently is offered publishing opportunities, and who does the following:

1. Asks A to write an original article for Z, requiring extensive research, that is published in a respected scholarly venue under Z's name exclusively.
2. Asks B to write an original article, requiring extensive research, that is published in a respected scholarly venue, giving authorship to Z and B in that order.
3. Asks C to lead the research and writing of an original article among a team of researcher-writers D, E, and F, that is published in a respected scholarly venue, giving first authorship to Z, and then C, D, E, and F based upon their contributions and effort.
4. Several years after co-authoring a scholarly book with G, H, and J that is published by a respected academic press, whereby authorship is attributed in the order Z, G, H, and J -- Z asks G, H, J, and K to prepare a second edition of the book, requiring substantial research and writing, and maintaining the original order of authorship without adding K.
5. Asks L to lead the research and writing of an original article among a team of researcher-writers M, N, and P, that is published in a respected scholarly venue, giving authorship to L, M, N, and P based upon their contributions and effort, and Z as last author.
6. Q, R, and S conduct original research and prepare a manuscript for publication in a special issue of a scholarly journal. Z is guest editor of the special issue. Authorship is given to Q, R, and S based upon their contributions and effort, and Z is last author.
7. Z conducts original research and writing with T and V and their work (ZTV) is published in a scholarly venue, giving authorship to Z, T, and V in that order. A few years later, Z repackages ZTV and publishes it as sole author in a separate scholarly venue. The credits acknowledge that portions of the new article were drawn from ZTV, though neither T, V or the publisher of ZTV were consulted about the repackaging.
8. Z conducts original research and writing, which is published in a scholarly venue with Z as sole author. A few years later Z repackages the original article and publishes it in a separate scholarly venue. The credits acknowledge that portions of the new article were drawn from the original, though the publisher of the original article was not consulted about the repackaging.

In addition to the above, Z generally provides some comments and edits to the final drafts. None of the researcher-writers A through V is a studcnt; they are all professional researchers and academics.

Do any of these scenarios constitute plagiarism? Why would any of the researcher-writers A through V permit these outcomes?

Comments are very welcome, as always. Each reader's thoughts regarding whether the 8 scenarios above (any or all) are acts of plagiarism, are encouraged.