I am digging into the ability further customize some functionality around the contact role, but this join between customer & contact is feeling a bit mysterious.

I would start with a contact search and then pull in a column for the customer name

Indeed, that just works and delivers the report I requested.

I've created custom list fields on contact records and then just assign the values you want to have as a selection. It's under Customization > List, Records, Fields > Entity Fields and then just check the box for Contact

right, we've figured that out but those values would show up on every customer to which that contact is attached which isn't what we were after ... anyhow, after much discussion here we are probably going to highly discourage attaching a contact to multiple customer records. SCA logins for e-commerce distinguish customer choice by have a different password for the same e-mail address which we believe to be a bad user experience. So every contact needs to a distinct e-mail address.