Sounds to me that what you’re wanting is a show trial: a trial of Khadr, without regard to whether he will be convicted, but because it’s politically important to show support for the military by charging him.
We don’t do show trials in Canada.
The Crown prosecutor has to be satisfied, based on the evidence and the applicable legal principles, that there is a reasonable likelihood of conviction. It that test is not met, and the Crown does not think that there is a reasonable likelihood of conviction but proceeds to trial for other motives, such as a need to “support the troops”, then the the Crown prosecutor would be committing the civil tort of malicious prosecution. The Crown would also likely be breaching the accused’s Charter rights.
We don’t institute criminal trials in Canada “to send a message.” A criminal trial only occurs if there is a reasonable likelihood of conviction.
The courts start with the law as declared by Parliament. Here, Parliament has not said “any Canadian fighting in the same theatre as Canadian troops commits treason.”
Parliament has instead used a much narrower test. A person commits treason who:
Being engaged in fighting with a Canadian ally in the theatre is not enough; the person must have assisted an enemy at war with Canada, or armed forces which are engaged in hostilities with the Canadian Forces. That is an important restriction on the offence of treason, which historically has been used for political purposes. Instead, Parliament has given a clear and objective test, that cannot be met simply by saying “enemy” and “same theatre.” Absent evidence that brings Khadr within the language of the offence, quoted above, a Crown Prosecutor should be very wary of allowing a charge of treason to proceed.
A few comments.
First, Canada has signed the International Convention against Torture, and has implemented it by making it a criminal offence for a person in authority to torture a prisoner in their control. It is reasonable for the courts to conclude that Parliament would not want evidence obtained by torture by a foreign state to be admissable or used in a Canadian prosecution. This is a much more basic principle than simply the paperwork relating to a detention. If a person has been tortured (and there is a substantial body of evidence that lengthy sleep deprivation may amount to torture), then the evidence will likely not be admissible in a Canadian court, even if the detention was under the control of a foreign state.
Second, Parliament has provided that young persons charged with a criminal offence get stronger procedural protections than adults. Courts are required to implement that clear policy directive from Parliament. If a person under 18 is charged with high treason, then that person has procedural protections greater than those of an adult.