Can a (stapler) PhD thesis comprise multiple unrelated papers?
This is dependent on your supervisor, the comittee and the referees (in the systems I know, almost exclusively the supervisor). Moreover, I would expect from any sane supervisor that they know the projects you are working on and at least warn you if you work too much on topics which can not be put into your thesis.

Most of the time (if your supervisor agrees) and you write an introduction outlining connections between the topics, you are fine. I have seen theses where the introduction was "We present some.papers in the area X, here is an introduction to X" and everything was great. It's just a formality.