The carriers do in fact have a say in the upgrade schedule. I know that (at least for a little while) they stopped being able to release updates quite as quickly once they released the iPhone for Verizon in the US.

And yes, if you buy your phone directly from Google, you'll get your updates directly from Google with no delay (other than their staged rollout process).

Yes, it's Google rolling out upgrades to users directly.

(I have never owned an Android phone other than the Nexus series.)