Exch 2016 Hybrid to Exch 2019 Hybrid /TenantOrganizationConfig queries

Some good questions. It's a bit confusing where it says "For existing environments, you don't need to use the /OrganizationName and /TenantOrganizationConfig switches." Obviously if you have a hybrid in place you have an existing environment, so the two comments seems to counteract each other.
I don't think schema preparation is skipped. The article says that "schema extension is performed implicitly by running the /PrepareAD switch."
It would be good to get this cleared up by Microsoft. I would generally recommend re-running the HCW after any changes unless you've customised any of the objects created as part of the HCW process.