... CDIC guarantee, which is backed by the Government of Canada.
Are you sure about that? CDIC is a crown corporation that insures deposits. Premiums are paid to CDIC, which builds up reserves in case of a bank failure. Now, if one of the big five were to fail, there would most like not be enough reserves to pay out, so there would be political pressure on Ottawa to make the payouts whole, but there is no guarantee the federal government would top them up. I would assume the provincial associations would work in a similar fashion.
(This is similar to the pension benefit guarantee fund in Ontario. With the recent issues with the car companies, the PBGF does not have enough money to cover the full amount of pensions, so there was talk of the Ontario government having to top up the fund in that case.)