if you had a top menu then the activation is done and that little snippet of avia_meta_header is unnecessary:
this you can erase : add_filter( 'avf_execute_avia_meta_header', '__return_true', 10, 1);

the css will depend on the position of top menu – this here is for left position:

#header_meta .av-rotator-container {
    font-size: 24px !important;
    position: relative;
    left: 50%;
    float: left !important;
    transform: translateX(-50%);
    top: -10px;
}

.av_secondary_left .sub_menu {
  width: 80%;
}

#header_meta span {
    font-size: 30px;
}

there is a little error in the textblock.php

https://kriesi.at/support/topic/4-6-3-1-issues/#post-1144794

here is a corrected textblock.php : Link

If you do that all testings : do you allways refresh all caching tools ( including autoptimizer , enfold merging etc.) ?

small addendum: sometimes there are other elements between the mfp wrapper and the wrap_all. So wrap_all is no longer a direct neighbour.
Therefore you should rather use this one:

 /* instead of the code ( direct neighbor ) */
.mfp-wrap + #wrap_all {
  position: fixed !important ;
}
/* this here ( General Sibling Selector ) */
.mfp-wrap ~ #wrap_all {
  position: fixed !important ;
}

so it is now:

 .mfp-zoom-in.mfp-ready.mfp-bg, .mfp-zoom-in.mfp-ready .mfp-preloader {
      opacity: 0.8;
  }

  .mfp-content {
    position: fixed !important;
    top: 50%;
    left: 50%;
    transform: translate(-50% , -50%);
    overflow: hidden;
  }

  .mfp-arrow {
    position: fixed !important
  }

  .mfp-wrap ~ #wrap_all {
    position: fixed !important ;
  }

  img.mfp-img {
    max-width: 70vw;  
    padding: 10px 0 !important;  
  }

PS : it is clear that if you put it in a media query only for screens bigger than 768px – that the code does not work below this screen-width

on : ‎⁨ ⁨borlabs-cookie⁩ ▸ ⁨classes⁩ ▸ ⁨Cookie⁩ ▸ ⁨Frontend⁩ in Frontend.php

if I comment out the enfold lines the function runs ! :

line 116:

// Enfold
if (function_exists('avia_register_frontend_scripts')) {
    add_action('avf_sc_video_output', [ThirdParty\Themes\Enfold::getInstance(), 'modifyVideoOutput'], 100, 6);
} 

so there must be something in the Enfold.php that blocks selfhosted Videos.
borlabs-cookie⁩ ▸ ⁨classes⁩ ▸ ⁨Cookie⁩ ▸ ⁨Frontend⁩ ▸ ⁨ThirdParty⁩ ▸ ⁨Themes⁩ ▸ Enfold.php

no one hampers you to calculate other width: f.e.

for my example page:
because it is a grid-row after the slider the calculation it a bit different from color-section as follower.

@media only screen and (max-width:989px) {
  #united_slider .avia-slideshow.av_fullscreen {
      width: calc(100vw - 16%) !important;
      height: 45vw !important;
      left: 50%;
      transform: translateX(-50%);
  }
}

this is for your problem the best construction.
Take only a fullscreen slider – set its screenoptions to display everywhere !!!

:https://webers-testseite.de/slider-depending-on-screenwidth/

there is even for you a better moving script for the slider before the header container with a little performance optimizing debounce function.
Enfold itself has a debounce function included but i didn’t know how to use it for this function.

The autop function and the removal of line breaks is a WordPress peculiarity. I’ll just show you how to get around it. That’s all.
The one with the small plugin is to be set up once, then also the one who manages to make a font bold can insert such a linebreak into the text editor, which doesn’t disappear.

well i do not see your site – but if it is not to complicated – maybe only text – with links
then you can use the headline-rotator shortcode as Rikard said and maybe a hook to embed this.

https://webers-testseite.de/guenni/datenschutzerklaerung/
it is simply done via child-theme functions.php and on styled before on a post or page. to get the shortcode:
here just for the page-id-3:

in this manner: echo do_shortcode(“…”);

add_action('ava_main_header', function() {
if(is_page(3)){
echo do_shortcode("[av_headline_rotator before_rotating='' after_rotating='' interval='5' animation='' margin='' margin_sync='true' tag='h2' size='36' align='center' custom_title='' av-medium-font-size-title='' av-small-font-size-title='' av-mini-font-size-title='' alb_description='' id='' custom_class='' av_uid='av-k1gw6wo0' admin_preview_bg='']
	[av_rotator_item title='today we offer abc' link='' linktarget='' custom_title='' av_uid='av-it587']
	[av_rotator_item title='but the whole week it is' link='' linktarget='' custom_title='' av_uid='av-egknj']
	[av_rotator_item title='another line to present' link='' linktarget='' custom_title='' av_uid='av-6u493']
[/av_headline_rotator]");
};
});

here is a little description on how you get this shortcode: Link

From Participant to Participant:

I am waiting since 47 hours now

Actually, I wanted to look at your code; but when I read something like this, I move on to the nearest problem.

If you don’t make yourself a slave to Performance Test Tools, then I think a little movement on the start page is good.
On my site I even have a small self-hosted movie running, which also works on the mobile phone!

this is my function for him sofar- maybe a mod could tell me how to use the included debounce resize function on enfold for it:
( the selector has to be more specific – maybe an ID or Custom Class – but that could be done later)

function slider_move(){
?>
<script>
(function($){
	var fullscreenSlider = $(".avia-fullscreen-slider");
	fullscreenSlider.clone().detach();

	$(window).on('load resize', function () {		
		var winWidth = $(window).width();
		if (winWidth < 990) {
			$("#main").prepend(fullscreenSlider);
		}
		else {
			$("#wrap_all").prepend(fullscreenSlider);
		}
	});
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'slider_move');

then you should remove the script: …

(function($){
        $('html.avia_mobile #fullscreen_slider_1').remove();
        $('html.avia_desktop #full_slider_1').remove();
})(jQuery);

we had to find a way now for your slider that it is only inserted before header on screenwidth dependency.

Doesn’t seem to have been in a hurry.