A preliminary hearing or preliminary inquiry is merely a process in which the Crown presents enough of its case to satisfy the Court that there is some evidence to support the matter moving forward to trial. An accused does not testify and there is no determination of guilt or innocence, just an assessment of whether some evidence exists upon which a trial could be based. This is also an opportunity for the defense to test the Crown's evidence prior to trial.
Ultimately, it is the Crown's burden to show that some evidence exists, so they would call whomever they deem necessary to present enough to meet the test for moving forward. There is also leeway for the defense to indicate who they would specifically like to hear from at a preliminary hearing. The defense can indicate that they only want to hear from the complainant, however the final say in who is called is up to the Crown as again, it is the Crown who must meet the burden of satisfying the Court.
The Crown will often subpoena a number of witnesses, or more than just who the defense wants to hear from just to be safe, but they may not necessarily have them testify.
The fact that the Crown has subpoenaed more witnesses than just the complainant really means nothing other than they are simply covering the bases to ensure that they can put sufficient evidence forward if required. I would not read anything into this at all.
In fact, I always welcome more witnesses at a preliminary hearing as it gives me an opportunity to test them out, run cross-examinations so I know how they'll be at trial and it provides an opportunity to discredit untruthful witnesses if their stories change from the prelim to the trial.
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