Primary email is used to login to firefox account if you lost access to reset password for that email there nothing more that can be done. It's consider a lost account.

I still have access. I'm worried about privacy. I don't want my old email that "used to be my primary email" somehow still tied to my account. How do I check that it is?

If it is, then why? How can I delete it? I have good reason to think that it is given my attempt to login with it returns anything other than "you must create a new user password/profile."

Did you try to change the email in Firefox accounts yet? The Primary account is used to sign in - you need to change the Primary account email login if you don't want to be locked out or loose access to firefox account. And what happens to that email beyond using as FireFox login - firefox has no control of that.

I had "made" a second email (added a second email in my firefox account), then turned the second email into my primary email, then deleted the "old primary." Now I'm just paranoid that the old email is still wondering around firefox servers somewhere...

FireFox has no backup of such old email logins once you change it - the new one is set as the login email. That's all I know from my limited usage it's used for only. They don't send out emails or contact you through the email. Only other way is for you to that old email and have that email service delete that account.