WP24

Thanks for reporting this issue, an update has just been released to fix that.

You could change the style via custom CSS:

.wp24-dc input[type=submit] {
	color: #000000;
	background-color: #ff0000;
}

Is there any error message appearing?
Which WordPress version are you using?

As u stated this feature of an alternative set of price and link would not be used by many users. Therefore adding a hook would be the better solution.

Something like this to manipulate the result on server side (e.g. in functions.php of the theme):

function change_whois_result( $whois_result ) {
    if ( in_array( $whois_result->tld, array( 'com', 'net' ) ) ) {
		$whois_result->price = '1.99 $';
		$whois_result->link = 'https://ww.wp.xz.cn';
	}
    return $whois_result;
}
add_filter( 'wp24_domaincheck_whois_result', 'change_whois_result' );

Since the domaincheck file for the ajax request uses only a short wordpress init, hooks are not possible. Hence a new option “Enable Hooks” could be added, so that the whole wordpress framework is loaded. This will slow down the whois requests a little bit.
You colud test this removing the following line from assets/inc/domaincheck.php:
define( 'SHORTINIT', true );

If you are satisfied with this solution it could be added in the next version of the plugin.

To check multiple TLDs the option “Check All” must be enabled in the general settings.
With selection type “Drop-Down List” you then could choose “all”, with “Free Text Input” just omit the TLD and you get the list like in the screenshot.
You need at least version 1.2.0 of the plugin, as the check all feature was added there. (The installed version is shown under “About”.)

Marked as resolved.

Have you solved the problem already? On the page you posted nothing seems to overlap.