This is complicated because the transaction occurred in Canada but the case was heard in Chicago. But the argument is the same as forgery: the court may not be able to force Doig to authenticate it, but if the officer can prove Doig sold him the painting under the pretence of being his then Doig has sold him a forgery (selling a claimed Doig which truly is not) and is liable for it. It does not matter that the name Doig did not mean anything back then, because the forgery constitutes a loss of future earnings.
Of course this is all hypothetical because Doig obviously did not paint this given what has come out in court.
The problem with allowing illegally obtained evidence to be admissible is that it sets a legal precedent. There are no one -offs in common law countries. The DA can use this case as an argument next time, effectively incentivising police to breach protocol in other more egregious ways like tapping into your phone calls without reason.
It saddens me deeply that Chahal is getting off on a technicality but ultimately there is no such thing as an objective spirit of the law. All we have is the literal text - the letter of the law - that's in the books and two sides arguing the best way to interpret that.
Writing warrant laws that protect good citizens and good citizens only is impossible. Sometimes bad people slip through the cracks as a result. It's a tragedy, but not one that is easy to remedy.
Do you mean in reference to "knock back"? I think its supposed to be knock back like one would with a beer - as in, consume readily - rather than knock back as in knock away or disregard.
Of course this is all hypothetical because Doig obviously did not paint this given what has come out in court.