I think I see what you mean, though maybe I'm misunderstanding still.

The answer (from my experience at Etsy, anyway) to questions like:

> how to modify the schema of the events going into B to add a specific piece of data,

was "Run schema_modifier", the docs for which were frequently obsolete. Figuring out how to "run schema_modifier" was usually a very frustrating experience in that particular instance, whereas unwrapping the actual problem was usually less frustrating.