Expected undergraduate semesters and requirements for graduate students
I have been wondering recently about how graduate schools and in general anyone who reviews my academic record views coops.


Coops are fantastic.  Especially for programs in computer science / applied math / data science, relevant industry experience is a strong advantage in graduate school applications.


  If the student takes more than 8 semesters to graduate, is it a problem?


No.  Nobody cares.  Even if someone on a grad admission committee noticed, the longer time would quickly be explained by your coop semesters and your change in majors.


  Should the student (if they can) extend their studies by an additional semester so that they graduate in the spring, or is a fall graduation equally acceptable.


Acceptable to whom?  If you mean graduate admissions committees, then I repeat: Nobody cares.  In particular, graduating in the spring is no barrier to starting a graduate program the following fall.  (Of course, you have to eat, but that's a separate issue.)


  Do grad programs accept students to start in the spring, or only the fall?


Some do, some don't.  Every grad program is different.

One last point: You mention coops, GPAs, and program reputation, but those are not the most significant bits.  Graduate admissions committees at top departments are primarily looking for strong evidence of research potential.  Assuming my quick Google search correctly identifies your university, you should have lots of opportunities for undergraduate research experience with world-class faculty.  Take advantage of them!  Start now!