Dear Richard,
1. Be default of the theme, the text in the slider is pre-built so it cannot be changed. Instead, you can update new slides in the slider by following
the path below:
Appearance > Customize > Featured Section Options -> Enable-Feature Section ( Check the box)
Appearance > Customize > Featured Section Options -> Slider Selection -> Add New Slide

2. The widgets should work well. Please make sure you remove the unnecessary plugins from your admin panel which can cause this issue.

Best Regards!

Dear Julien,
Such issues are common for the iOS devices as Safari browser still does not support background-attachment: fixed; property. But it should not appear on the Android devices.
But you can add the following code in Additional CSS for removing the blurry image from all mobile devices.
Appearance > Customize > Additional CSS

@media(max-width:767px){
    .inner-main-title{
       background-attachment: scroll !important;
   }
}

(Note: it will change in all mobile devices)

Best Regards!

Dear Abhianub,
Please follow the path below to add widgets in the home main content area.
Appearance > Customize > Home Main Content Area
You can add widgets there.
Best Regards!

Dear Emmanuel,
Please check your inbox.
Best Regards!

Dear Gbenga,
You will require to create a child theme. Please check out the link below to build a child theme.
https://codex.wordpress.org/Child_Themes
Best Regards!

Dear Julien,
Please add the following css code to make title visible in mobile devices.
Appearance > Customize > Additional CSS

@media screen and (max-width:767px)
.image-slider-wrapper .slider-content .image-slider-caption {
   display: block;
}

And the CSS code written for removing border-right is correct so no need to rewrite.

Best Regards!

Dear all,
For debugging process, please deactivate your installed plugin one by one then review the bug. If then also bug arises then also please feel to contact us.