2. To force the examiners to tick the box asking: "Is part of this thesis publishable?"
However, if you are not citing, paraphrasing or reviewing the field, there is no place for any references which have nothing to do with your thesis-theme.
Another approach is to bring your papers up to date, unify their approach, and use them as chapters in your thesis-book. But, beforehand you have to plan a modular approach for this to work. The first sentence of each of these chapters will begin: "This chapter is an updated and revised version of Aminu (2xxx) ...."
Yes for sure. Thesis is always guided by supervisor and advisers. Normally a new thesis is a continuation of what was done before, be your own, your supervisor suggestion or others. So I see nothing wrong by citing previous works specially those related to your lab.
My own preference is for self-citation which amplifies points being made in the text in front of one: in other words, these citations direct one to previously published work in order to provide additional evidence or argumentation. I don't like it when scholars simply skip arguments in the text by saying they have already been discussed elsewhere (i.e. in earlier publications).
In my opinion, if there is quality literature and enough for references, leave yours out. Otherwise, if the researcher already has a good trajectory, being a reference for others, I understand that it should be included.
There should be no issues if the previous material that you're hoping to cite has already been through the peer-review process and published via journal or conference. All theses and dissertations are done under the auspices of a committee whose responsibility is to ensure the quality of the final document and its contents. If they're fine with it, then there should be no problem.
Yes, all published article which has past the criterium of other doctors can be a solid basis for a thesis. If the article is of the same author, it is a good recommendation for the candidate to doctor.