How do I make my rescue email my primary email?

You should be able to edit / remove the rescue address as described on that page :

1. Navigate to appleid.apple.com using your web browser.
2. Click "Manage your account"
3. When prompted, sign in using your Apple ID and password.
4. Click Password & Security
5. You'll be asked to answer 2 of your 3 security questions before you can make any modifications. If you are unable to remember your answers, you can choose to send an email to your rescue email to reset your security questions.
6. After you've validated your identity by correctly answering your security questions, click Editto the right of your rescue email address.
7. Once you've finished editing your rescue email address, click the Savebutton directly below the email field.

When you've done that you should be able to use it as your main email address

that doesn't work for me.

still comes up with Can't use rescue email as primary

Did you try logging out of that site between removing it as your rescue address and putting it as your main address ?