Some good patterns here. An Event with a callback (what you're calling [context protocol](https://github.com/webcomponents-cg/community-protocols/blob...)) I think will prove useful.

My main gripe with web components is that while they expose the first half of the tree traversal (from the outside in, via connectedCallback), they don't provide access to the second half, after child elements have been created. (akin to Initialize and Load events from ye olde .NET)