Two issues here wordpress and email.
First wordpress, use rename in your ftp program to put the domains back the way they were before, rename the domain directroies DomainA.com to DomainTemp then rename DomainB.com to DomainA.com then DomainTemp to DomainB.com
Now login to the worpress Admin and change the url for your site there. You will get an error message, ignore it but doublecheck your entry.
Now repeat the directory renames above again to switch them back. Now you should be able to log into WP admin under the new domain.
on the email side of the issue, there is a “mail mirror” function on the Custom MX / Gmail tab in the Panel. I belive that’s what your trying to set up.
To see the “mail mirror” option edit any domains on the Custom MX tab in the panel.