Hello, @Maheeb Sons,
1- Great.
2- I made a video for you to know how you can create different headers for different pages: https://streamable.com/acy9is
3- Great.
4- It is because the form is coming from a plugin and the plugin doesn’t work on AJAX so that’s why the page is loading when you click on the next step. For this, you have to contact the plugin author or check for any other plugin that suits your requirement and use it at your own risk.
5- I have checked the menu link that you shared with us it is working fine I made a video for you there is no error see video: https://streamable.com/sx09no
You can add multiple menus on the header it will work fine as you can see in this video: https://streamable.com/e33e2m
6- This is not the CSS, this is the custom PHP code to make a plugin compatible with the theme and you can paste it on the child theme function.php file.
Regards 8Themes Team.