Buying the physical Collector’s Edition doesn’t grant access to the Dragonflight expansion, at least not from what I can see of the product page. That is currently only included with the digital purchase on our shop (the idea being to allow access to Dragonflight if you don’t have it already).
Traditionally, when an expansion is released the content from the previous expansion is added to the base copy of the game. For example, when Dragonflight was released all accounts received access to Shadowlands content regardless of if they purchased it or not.
As far as I know, there is no email sent when you purchase the Collector’s Edition that includes a key of any kind. The key should be in the physical box that should arrive around when the War Within launches.