Dear Hippie1975,
By default of the Mercantile theme, the feature is not available in the theme. Hence, it will require theme customization.
Best Regards!

Dear Julien,
Please follow the path below and disable the sticky menu option so the page won’t scroll on the top.
Appearance > Customize > Header Options > Menu Options
Uncheck the box and save the page.

Best Regards!

Dear benpaddlejones,
For the mobile menu button to be centered below the logo.
Please add the following CSS code in Additional CSS
Appearance > Customize > Additional CSS

@media (max-width:1024px){
    .navbar-header {
        display: flex;
        flex-direction: column;
        justify-content: center;
        align-items: center;
    }
    .navbar-toggle {
        order: 2;
        margin-top: 20px !important;
    }
    .custom-logo-link {
        order: 1;
    }

}
    

In the debugging process, we found that parent theme style is loaded before boostrap.min.css which is not a valid way to load style. Here is the actual hierarchy of style loading. “bootstrap.min.css” should be load first then parent theme style.css then after child theme style.css.

Note: If you feel difficult to create child theme properly then we recommend to use )( https://wordpress.org/plugins/child-theme-configurator/ ) plugin to create a child theme.

Best Regards!

Dear acp2018,
If you can provide us your site URL, we will observe and can provide you with possible solutions.
Best Regards!

Hello thegreenlight,
You can customize the header.php (event-start/acmethemes/hooks/header.php) for the header part of the site.
Note: Your all listed queries fall under header.php file.

Dear Tron99,
Could you please provide us with the URL of your site so that we can provide you with a possible solution?

Best Regards!

Dear Brian,
We recommend you use the given plugin for updating the theme.
https://wordpress.org/plugins/update-theme-and-plugins-from-zip-file/

Best Regards!