Sometimes, there is little or no question about the diagnosis and one can be quite confident. Other times, a practitioner will provide a "provisional" or "working" diagnosis, which may even include certain "rule out" suggestions.
It really depends on how clear cut the symptoms are and whether there are sympsoms of more than one issue or disorder.
It depends upon the situation. In the US, one must have a certain level of licensure to diagnose. Right now LCPCs, LCSWs, Licensed Psychologists and Psychiatrists (of course physicians) can diagnosis. Ethically, if a person has not had the training to diagnosis then they should not be doing so as it would be outside of their area of competency.
I don't know how things are done in Canada. As David said, a therapist can diagnosis provisionally, and the personality disorders can be used but instead of the full disorder, one can use PD features.
BG...Master level therapists in the US can diagnosis. Physicians can but I think they should have mental health training to diagnosis. Psychologists tend to have more experience in terms of diagnosis than other master level clinicians.