UK Visitor extension with PR Application (Common-Law)

Hello,

I was wondering if anyone could help? Two months I handed in an in-Canada application for PR, which I am still awaiting confirmation that my common law has been approved to be sponsor. I am a British Citizen who is living with his common-law partner in Canada, though as of mid-October my six months is up and I will need to extend my stay if I wish to remain in Canada. I have a few questions regarding this:

1) Can I extend my stay (gain another six months) at the border with the US asi tis only a half hour drive away? I believe this is called "flagpoling"
2) If I were to extend my stay, would I be eligible to fly back to the UK, and return a couple of weeks later on the same visa? I'm just thinking ahead for christmas and I'm not sure if the visa suggested on the CIC website is muliple entry.

Many thanks in advance,