by the way the code couldn’t work because you changed the markup totaly – the ID etc .
there was first an ID of: #banner – now it is : #av_section_12
selector now is #av_section_12 .flex_column.av_one_full

So if you like to see what happens :

#av_section_12 .flex_column.av_one_full {
    background-size: cover !important;
    background-repeat: no-repeat !important;
    background-position: center !important;
    background-color: rgba(0, 147, 255, 0.5) !important;
    background-blend-mode: soft-light !important;  
}

make a custom link to your menu as last link on the menu!
Link Target – you know it better
Link Label : Order Now
Menu Style : Button Style Colored
Custom Class: menu-item-avia-special

see her: https://webers-testseite.de/ – look what happens to the button on opened hamburger !
styling of the button is quick css work.

i think – no – not only on GDPR (DSGVO on german spoken Countries) Reasons.
With a selfhosted video this could be done.
https://webers-testseite.de/mute-unmute/
Besides these infos – the browsers do it all their own way.
Look (Listen) here on Enfold Fullwidth Easy Slider – it starts muted – and you can turn up the volume on player controls. https://webers-testseite.de/vimeo/
Listen here in a codeblock and with iframe – but it does not autostart on firefox (chrome does) : https://webers-testseite.de/vimeo-2/

Thanks Ismael : what i mean is if i put something in the copyright field – it will change the copyright for all pages where this image is used – and that makes sense – this is a global property.

An Alt or Title attribute can differ depending on how and where it is used. Just when I think of the use on multilingual pages. Yes, I know that WPML achieves this by providing an additional plugin (Media Translation) for the different languages. But it has the disadvantage that it creates duplicate images and inflates a backup.

If you create a gallery ( masonry gallery ) there is that point where you can insert under each image a “Captiion” – this will only affect the caption for that specific gallery – so it will be added specifically only here to the image. There I thought I could create two additional input fields, which would add the attributes here similarly only for the use of the image.

Well this is my layout:

but after that – open the cell with the easy slider – it seems that the cell got a padding on default of 30px – you had to remove this.
https://webers-testseite.de/grid-row-with-slide-show/

but i guess that images in landscape will be best. And text on the other cell is less high than the slide-show

The crux of the matter is that one can distinguish same selectors by counting.

But be careful because it is counted over all.
So if div, p, ol, ul tags appear , then it is counted over all.
Sometimes it is advisable to go over nth-of-type: The :nth-of-type(n) selector matches every element that is the nth child, of a particular type, of its parent.

https://css-tricks.com/the-difference-between-nth-child-and-nth-of-type/

see the markup from: “Let’s say our markup changed to this:” there you can see that nth-child will not work as wanted.

and if you know what you like to have use a filter : https://kriesi.at/documentation/enfold/masonry/#change-the-sort-order-of-masonry-elements

or this child-theme functions.php snippet ( or is it deprecated in 4.6.3.1 ? )
https://kriesi.at/documentation/enfold/blog-post/#how-to-add-an-orderorderby-option-to-the-blogpost-sliderportfoliomasonry-grid-element

Many hosters are now forced to switch to higher php versions. Most notify you that the 5.6 versions will now be disabled, but not all. I don’t remember when the enfold versions needed newer php versions higher than 7, but maybe that’s the reason.

Then: did you disable Gutenberg (Block Editor)? I only use the Classic Editor setting with Enfold:
Enfold (Child) – Theme Options – Select Your Editor

the first thing when activating Enfold as theme i do insert this to my child-theme functions.php:

function admin_head_mod() {
  echo '<style type="text/css">
  #avia_quick_css .avia_description {float: none;margin-bottom: 20px !important;padding-left: 0 !important;width: 98%}
  #avia_quick_css .avia_control {float: left;max-width: 98% !important;width: 98% !important}
  .avia_footer_links li {float: left;padding-right: 20px}
  .avia_footer .avia_footer_save {float: left}
  .avia_reset {pointer-events: none !important }
  #avia_options_page { max-width: 3000px!important}
  </style>';
}
add_action('admin_head', 'admin_head_mod');

or in that code abbove set .avia_reset to display:none

it disables the reset button and let the quick css field and the whole options page grow to max width.
The quick css is to narrow for some selectors.

sorry : both sites
a) https://www.bellarinepeninsulaaccommodation.com.au/
b) https://www.greatoceanroadaccommodation.directory/

I’m sorry I’m saying this now, you think you should play it safe and start over.
So-called Nulled Themes can also put code into other folders (of course also into the wp-content folder etc.).
For me the risk would be too high. Therefore : even the database should be completely rebuilt. And also upload a new WordPress. Sounds paranoid, but it would be too unsafe for me to invest too much work and then be blocked by Google because the site sends tons of spam.

Please do upload a complete new Enfold Folder like here: https://kriesi.at/support/topic/some-hints-and-advice-to-update-enfold/#post-1056107

Because: Pirated (Nulled) Themes do often includes malware or spyware.
so to be sure that there is no additional code in your enfold folder – this was my advice.

PS both sites are still on older versions: the one on 4.0.5 the other on 4.4.1

Yoast does it right if it does not count the backgrounds as images. these are in css not in the DOM as img tags.
Yoast only looks if all img tags have alt attributes.