I admit I was too lazy to read it all at 03.15am, but just before I dig in (and it will also be a reminder to read it tomorrow if I have a notification :)), what is your benefit from protected static over the regular public?
Hopefully, when you read through, you'll see what's happening... we're actually leaving the conventional scope method as it is (public), but we're adding a new protected instance method (not static) and calling that from our scope instead.
This then lets us use the magic methods to access our protected method from a static and external context, allowing the 'scope' to be used inside a constrained, eager-loaded relationship subquery.