Based on this description, it sounds like someone walking past your unattended desk and bent on disrupting your day but not stealing your data, could enter in a garbage password into the lock screen a few times and lock you out of your own laptop.

I guess the same also works for cloud accounts as well. I remember, back in the mid-2000s, trying to log into my hotmail account (never having failed to log in before) and getting a "locked out due to too many bad passwords". So someone, only knowing my user account name (which was the same as my email address), locked me out of my own account. The problem was, I couldn't remember what my recovery accounts were (I eventually figured it out).

Is there really no supported model for this scenario? Surely the point of an iCloud backup is that you can restore from the cloud rather than do a local hack to try to regain access to locked keychain db.

What happens if you just set up the device as a new machine and login to your iCloud like normal?

Good information to have. I was surprised by step 2 though (rm login.keychain-db). How can you be absolutely sure it doesn't contain anything important and you won't need it later?

I'd probably opt for a more defensive action here and just rename it (like the original reset did).

It Just Works™... until you don't want to take the default option. I'm sure your average user would just be SoL if going through this same experience.