My understanding is that they just have to prove beyond reasonable doubt that you did not take enough care because that’s an offence by itself. You have a civic duty to think if something may have been stolen. They don’t have to prove what you were intending to do.

Is that correct? That is true for certain very specific types of crime like money laundering, where bankers are expected to prove to themselves that the source of funds is legitimate. I don't believe such requirements apply to literally everyone, as such supply chain investigations can be arbitrarily expensive.

I believe it's correct. They're not expecting normal people to do supply chain analysis, but they are expecting people who see a ton of second hand drug-store products in a bag in a dodgy warehouse run by someone who doesn't really seem like a drug-store distributor to say 'wait a second that might be not 100% legitimate'.

It's not an unusual concept in common law. It's like manslaughter - you can have had absolutely not intention whatsoever to kill or harm someone, but still be held responsible for their death because you weren't careful enough.