- The source of Bob's idea does not matter.
- The act needs to be taken with the purpose of defending oneself.
- There is no "threshold" — reasonableness is judged based on all the circumstances.
The reasonableness of the source has no bearing on whether an act is self-defence
You ask (Question 1):
What is the threshold for reasonable source when one is being mugged or assaulted?
and then describe in much detail how Bob got the idea for a potential response in the future.
It doesn't matter that Bob got the idea for potential future actions from a "tough friendly worker" or a "street wise stranger." The source has no effect on a self-defence analysis.
You are describing potential self-defence
You ask (Question 2):
If Bob followed this perhaps realistically street wise stranger’s advice next time he found himself in such a situation, would he commit an offence?
The person claiming self-defence must believe in their mind that "a threat of force is being made against them or another person." That belief must also be based on reasonable grounds. They must also act with the subjective purpose of protecting themselves or the other person from that use or threat of force. And the act must be reasonable in the circumstances.
This all comes from s. 34 of the Criminal Code. It reads:
34 (1) A person is not guilty of an offence if
(a) they believe on reasonable grounds that force is being used against them or another person or that a threat of force is being made against them or another person;
(b) the act that constitutes the offence is committed for the purpose of defending or protecting themselves or the other person from that use or threat of force; and
(c) the act committed is reasonable in the circumstances.
You ask (Question 3):
Does defensive force have to be of a passive, deflective or evasive nature to be lawful?
The act does not need to be "defensive" per se. But the act needs to be taken with the purpose of defending.
Whether the act is reasonable depends on many factors, including the nature and proportionality of the response — there is no "threshold"
You ask (Questions 4 & 5):
Or could hitting someone with the objective of subduing them, knocking them down, incapacitating them, or of intimidating or daunting/deterring their accomplices still receive an excuse from the law given the contextual circumstances? Simply put, what is the threshold of permissible reasonable defensive force when faced with an assault?
Section 34(2) says:
(2) In determining whether the act committed is reasonable in the circumstances, the court shall consider the relevant circumstances of the person, the other parties and the act, including, but not limited to, the following factors:
(a) the nature of the force or threat;
(b) the extent to which the use of force was imminent and whether there were other means available to respond to the potential use of force;
(c) the person’s role in the incident;
(d) whether any party to the incident used or threatened to use a weapon;
(e) the size, age, gender and physical capabilities of the parties to the incident;
(f) the nature, duration and history of any relationship between the parties to the incident, including any prior use or threat of
force and the nature of that force or threat;
(f.1) any history of interaction or communication between the parties to the incident;
(g) the nature and proportionality of the person’s response to the use or threat of force; and
(h) whether the act committed was in response to a use or threat of force that the person knew was lawful.
Regarding the proportionality of the response, "a person defending himself against an attack, reasonably apprehended, cannot be expected to weigh to a nicety, the exact measure of necessary defensive action" (R. v. Khill, 2021 SCC 37).
All circumstances are relevant, including the other parties. There is no threshold. Ultimately, whether the act is reasonable is a question of fact left to the jury (or judge as trier-of-fact).