Hello,
I contacted the MailChimp for WP plugin developer and explained the situation. Basically, his plugin works with shortcode, not with html, which is why submit isn’t working. However, given that in the header it is not possible to insert shortcode, he advises as follows:
“The easiest thing would be to contact the theme author and tell him to wrap the header fields in a call to the following function.
<?php echo do_shortcode( $header_field ); ?>
This will allow the use of shortcodes inside the header fields, I’m sure he will understand the specific code that’s needed. It’s a change he can do in a few minutes.”
Is this something that would be possible to do?