Under a work-related ARC, your work permit is tied to a particular employer. So yes, you would need a new work permit with a new job.
If you decide to change jobs, the advice I've always read is to make sure you have the work permit paperwork going with your new job before you quit/get fired from your old job.
You must hold the same type of ARC (Work or JFRV) continuously for 5 years to apply for the APRC (permanent ARC). A student ARC is not eligible to meet the 5 year requirement.
I don't know about these last two questions; I expect others on this board and the National Immigration Agency would be able to answer your questions.