Or at the very least, is there a simple way to figure out which is the right record (join field) you should use for the recipient email?

Where is the email address that you want to use?

For majority of my workflow email triggers I use entity: email. but the email lives on the customer record. The I've tested the trigger with a free form address and it works but it does not work with the various record fields i've tried. Customer, Entity, Sales Order.

That is unexpected. How are you using Email as a trigger?

Sorry, im not using emails as triggers, I'm building custom emails for order confirmation, order shipped, order is ready for pickup. The creation/editing of sales orders and shipping fulfillments is what is triggering these emails.

@Doug Harrison good to hear. enjoy.