Hi lolliestick,
When enabling the "Optimize JS" option, I found that excluding the /plugins/system/t3/base/js/menu.js file resolves the issue, allowing the menu to function properly.
Also, you can add this CSS code to your custom.css file to improve the submenu item:
.t3-megamenu .span12.mega-col-nav .mega-inner {
padding: 10px;
}