If Canadians choose to pay reparations to assuage their collective guilt as the ultimate benefactors, that's up to them.
There is no civil law analogy about how it is 'supposed to work' as there is no precedent.
Not true. For instance: Kenyan victims of colonial torture win right to sue UK | CNN
Could just as easily argue that Canada acquired the entire liability on independence like in a corporate takeover.
Sure... let's let the UK make that argument in court.
It's silly though because the idea of suing countries for what happened centuries ago is incoherent.
Why would that be incoherent? Certainly the span of time doesn't make it easy to gather evidence, but if wrongdoing can be established and an ill-gotten asset still exists, why shouldn't this be acted on?
Just about everyone could sue someone else for some reason or another as all surviving groups are the descendants of conquerors, oppressors and slavers.
But we're talking specifically about issues of property.
If someone - e.g. a First Nations group - never consented to transfer of land, why shouldn't they be entitled to get that land back?
If someone - e.g. an enslaved person - did work on a land to improve a property that they were never compensated for, why wouldn't they be entitled to a lien on that property just like any other worker who improves a property and isn't paid?
Would it be silly for Britain to demand reparations from Italy, Germany, France, Denmark, Norway and Sweden as the successor states of their former colonisers?
I think the key word in your question is "successor state." Is the government of Germany the same entity that went to war with the Allies in WWII? The question is murky.
OTOH, the UK has had one continuous government without interruption since 1688 (or earlier, depending on one's point of view). Canada didn't exist as a state until 1867. From the Seven Years' War until then, the only entity that had ultimate oversight for most of what's now Canada from the Seven Years' War to Confederation was the UK. Even after 1867, the British government that ratified every Canadian law passed - including the laws that victimized First Nations people - right up until the Statute of Westminster is the same British government that exists today.