So if you are wanting to override multiple settings values, what you might do instead of the code above is to just override what the plugin thinks the setting is when it’s on the alternate feed. So you could replace that code/filter with this one:
add_filter( 'sendimagesrss_get_setting', 'prefix_override_sendimagesrss_setting', 99 );
/**
* Override the plugin settings for the alternate feed.
*
* @param $setting
*
* @return mixed
*/
function prefix_override_sendimagesrss_setting( $setting ) {
if ( is_feed( 'email' ) ) {
$setting['thumbnail_size'] = 'full';
$setting['alignment'] = 'center';
}
return $setting;
}
This should change the featured image (thumbnail) size and alignment from the plugin settings values, but only on the alternate feed.
Note that this is a different filter hook. Hope that helps!