You would have to manually Flash the US Firmware on your Canadian device, but unless you have a US SIM I don't think that it would give you eSIM.. but I haven't given it any thought.
US Firmware with a Canadian SIM & Carrier, pretty sure that wouldn't work either.
The reason you don't get the early build of Android is because there is no such thing. It's up to your carrier to push updates to your phone, and they don't want to do it for you until you're on their network. Google doesn't release an Android update for an early version of the OS unless it is a security update (such as the fix for the Stagefright issue), an update to fix issues in the release you have, or the release of a new version of Android (ex. Android 6.0). If your device has a vulnerability that Google wants to fix, they will release a patch for it. These are not security fixes as they are often critical for the device. The Nexus devices such as the Nexus 6 will often get released updates first, and then other devices will follow. Usually Nexus devices get the early version of Android as well.
