In Canada, must commercially sold mussels be purged and purified in UV filtered water for 42 hours?
In regards to the first of the two questions you asked, " [does] the quote beneath from Mussel Myths &amp; FAQs - The Cornish Mussel Shack holds true for Canada too?"


  Do I need to soak my mussels in water and oatmeal?
  No. This used to be done to help purge the mussel of any grit. All commercially sold mussels legally have to be purged and purified in UV filtered water for 42 hours. During this time most of the grit and sediment is released from the mussel.
  
  Do I need to soak the mussels and throw away the ones that float?
  Salt water mussels should never be soaked in fresh water as this will kill them, and as explained in the above answer there is no longer any need to soak mussels. This was done in the past to identify dead mussels. People thought that a floating mussel was a dead mussel, which is also is not true. A live mussel can often trap an air bubble when it snaps shut, and this is why some mussels float. To identify dead mussels see our page on preparing mussels safely.


Yes, it does to a point; but not in regards to food safety - just taste. In my experience (I live in Alberta), you should still follow those guidelines simply from a taste perspective because while generally cleaned and soaked prior to commercial sale no process is perfect at any level (think "pre-washed lettuce" found at Loblaws as well). I have found rinsing them as you listed helps with the taste. I do this with both the store bought mussels I cook, as well as the freshwater clams I catch (not quite the same process but very similar). 

Now, onto question two - I know in Toronto there will be a local fish market or seafood market that specializes in shellfish, call them. They will be happy to assist you in understanding the regulations as it pertains to the consumption of all kinds of seafood as some regulations also vary from province to province.