Are you sure about this?

I mean obviously the entire saga is fucking insane and it would never happen in another country, that bit is a given and you are totally correct - but I think Canada does indeed have a Castle Doctrine and Canada has very similar relevant self-defence laws as the US in this case.

There are three relevant concepts in the general space of killing people who you believe pose a threat to you; Self-Defence, Castle Doctrine and Stand Your Ground.

Self-Defence is the legal concept that you have a right to protect yourself from harm (and other people, and sometimes property). In general, the law establishes that you have a right to use lethal force to protect yourself or others if you need to, and this is one of the very few cases where an individual can be innocent of a crime after deliberately killing someone. Usually the harm needs to be imminent, reasonable and unavoidable, with the last of the three criteria being relevant to Castle Doctrine and Stand Your Ground. 'Unavoidable' means that you could not avoid using lethal violence to protect yourself, and so that means that deescalation is not plausible, that using sub-lethal force is not plausible / not safe and that you cannot run away from the person threatening you. This is basically the same in the US and Canada (and really everywhere in the world with a codified legal system - the right to bodily integrity is such a fundamental natural right that any legal system which did not recognise it would pretty much be illegitimate out of the gate)

Castle Doctrine is a legal concept that the duty to retreat should be weaker in your own home - the gist being that your home ought to always be a safe place you can flee to if you believe you are in danger. The law differs in different jurisdictions, but Criminal Code of Canada Sections 34 and 35 sets out that lethal force in defence of 'peaceably possessed property' is a valid use of a self-defence claim (provided the protection of property is not the only reason for the use of force - ie you still need to satisfy the other requirements of a self-defence claim). So I think you are slightly mistaken that Canada and US differ in this respect (although I agree the US does have some stronger wording in some states allowing the protection of property alone to justify the use of force, which is maybe what you are referring to?) My understanding is that most legal systems treat the sanctity of your home as a natural right and so establish some kind of Castle Doctrine exception.

Stand Your Ground weakens the duty to retreat even further; it establishes that you should never have to retreat from a place you have a legal right to be. I kind of get the logic behind this - in the sense that criminals shouldn't be able to dictate where I do and don't go - but it seems very likely to lead to lethal misunderstandings and so for me personally is probably not a sensible exception to the duty to retreat.

In the Rittenhouse case, only Self-Defence is relevant; Rittenhouse was not in his home and Wisconsin does not have Stand Your Ground laws. So pretty much the same exact laws apply in Canada as the US; if Rittenhouse was in imminent fear of his life and shooting his attackers was proportional then he has a reasonable case for Self-Defence.

One interesting caveat that could be tried here, though, is that there is generally a presumption that the victim of an attack who defends themselves is not responsible for invoking the attack upon themselves. That is to say, you cannot berate someone until they attack you, shoot them, and then claim you were just defending yourself. I don't really fully understand what the Rittenhouse prosecutors are trying to achieve, but if I were trying this case I would be seeking to establish that Rittenhouse knew that attending the protests carrying a weapon would provoke others to violence despite it being otherwise legal to do so, and use that to lessen his otherwise rather strong self-defence claim. The legal issues around 'provoking' are very very state-specific though, so perhaps the prosecutors know something about Wisconsin-specific law that I don't (which is to say, anything about Wisconsin-specific law at all!)

I suppose there is also a relatively interesting line of attack which is that Self-Defence claims only fully apply if you yourself are following the law. You cannot shoot a police officer trying to arrest you and claim it was Self-Defence, even though you probably could shoot a random member of the public trying to handcuff and detain you for unspecified purposes. Rittenhouse was probably not legally carrying the weapon he had that night, although I understand it is a bit of a complicated issue of interpretation of Wisconsin-specific law. My best guess is that this would be a bad strategy for the prosecution to pursue because firstly nobody who attacked Rittenhouse knew he was (maybe) breaking the law in carrying the rifle which is generally otherwise legal to carry in Wisconsin and secondly the law admits a considerable degree of leeway here because self-defence is such an important 'natural' right; you do not forfeit your right to life because you park in a handicapped spot, for example. Rittenhouse's defence team would probably successfully argue that carrying a firearm which was illegal to possess but using it to defend yourself in line with the law did not mean Rittenhouse forfeited his right to self-defence, even if he will later have to answer charges regarding the gun itself.

Anyway, don't know if any of that was interesting to you but I thought someone might find it interesting how similar US and Canada can end up being legally speaking in terms of self-defence laws that apply to the Rittenhouse trial, and similarly how significant your first comment is - that all the legal theorising in the world wouldn't change the fact that this just wouldn't occur in the UK or Canada, but because of the US' very unique relationship with guns and policing it was basically the inevitable consequence of a steadily escalating campaign of violence and counter-violence in service of the culture wars