For nav-bar centering. There are no universal solution because there are not enough divs wrapped over menu but you can use the following code for your website.
#main-nav ul li:first-child {
margin-left: 230px;
}
You can add it to your Child Theme style.css (if you use one) or add this to Theme Options → Other → Custom CSS.
2. You can try move menu below slider inside header-extensions.php. It might take a bit of coding to get this done. You can find this file in theme folder → library → structure.
3. Footer related code is located in footer-exntesions.php. You can find it in theme folder → library → structure.