that is what ismael said on your single page you inserted inside the content of that post a h1 heading.
(btw: this leads here to a duplicate h1 heading too)
The content of this page will then be taken over from the archive page. The title of the individual post is h2 within the archive page. However, the headings in the content are adopted as you have set them.
So if you now edit this single page (https://avaansmedia.com/when-should-a-startup-hire-a-pr-firm/ ) to convert this heading into a h2, then it will be the same on the archive page.

the example page above is an archiv page of personal – and yes the breadcrumb is h1 – but all the other blog title headings are h2.

That is why i ask if your page is similar to that – it makes me wonder. on the single posts – the titles are h1 but then breadcrumb tag is “strong”.
is there a link to your site that you can make public – as participant as you are – i do not see private content field here on board.

A page like that: https://kriesi.at/themes/enfold-2017/category/personal/

there all post do have h2 headings – and only the breadcrumb category is h1.

have you tried that already:

First – Mikes post is still right – but the need for the callback function is not necessary anymore – because there is now a filter for reaching the fixed background on lightbox opened.

add_filter( 'avf_default_lightbox_no_scroll', '__return_true' );

next: the code for that inline popup is correct but you had to know that the class open-popup-link had to be the trigger for the magnificPopup function.
On button Element the custom class goes to the button wrapper – not the anchor itself. – so the selector had to reflect that ( the class is parent of the anchor)

function inline_popup_every_avia_ID_you_like() { 
?>
<script type="text/javascript">
(function($){
$(window).on('load', function () {
	$('.open-popup-link').find('a').addClass('no-scroll');
	
    $('.open-popup-link a').magnificPopup({
		removalDelay: 500,
		type:'inline',
		midClick: true,
    });

    $(document).on('click', '.popup-modal-dismiss', function (e) {
      $.magnificPopup.close();
    });

 });   

})(jQuery);
</script>
<?php 
}
add_action('wp_footer', 'inline_popup_every_avia_ID_you_like', 999);

summary:

• the trigger is a link which is child of the custom-class: open-popup-link
• if you got a lot of these trigger links you can give that custom-class to the color-section etc. this indicates that all links inside goes to lightbox.
• if you want to link to another lightbox from that modal window – just place a button inside again with class: open-popup-link
• there is a second class – thats for a link inside the popup that could close the lightbox: popup-modal-dismiss
  nice to have for links in the lightbox that goes to an anchor link (f.e. one-pager sites).
• the triggered link should have now a manual link to an anchor with unique ID
• give to this element that goes to the lightbox the class mfp-hide (to make it invisible) and maybe the class white-popup (for prestyled styling of the lightbox by magnificPopup) and the ID of the button link

unterhalb des Logo Eingabefeldes gibt es einen Schiebregler für die Transparenz Optionen: “Transparency Logo Options”
den aktiviere bitte mal. Dann kannst du nun ein Logo für die Seiten mit Transparenz hinterlegen, sowie die Farben des Menüs festsetzen.

Achte bitte darauf, dass die beiden Logos die gleichen Abmessungen haben und auch sonst deckungsgleich sind. Dann ist der Übergang fließender.

well i would not use the hidden : .avia_hidden_link_text nor pseudo-containers because there is allready an existing “caption”
use the hidden strong container inside av-hamburger-box !
synchronize the media query with the hamburger “break point”
f.e:

@media only screen and (max-width:989px) {
  #top .av-burger-menu-main .av-hamburger-box strong {
    display: block !important;
    position: absolute;
    top: -10px;
    left: 50%;
    transform: translateX(-50%);
    color: #000; /*** edit this to your needs ****/
  }
}

you have to adjust the color for this and think of transparency option too.

for example you can use the variable color definitions:

@media only screen and (max-width:989px) {
  #top .av-burger-menu-main .av-hamburger-box strong {
    display: block !important;
    position: absolute;
    top: -10px;
    left: 50%;
    transform: translateX(-50%);
    color: var(--enfold-header_burger_color); 
  }
  #top #header.av_header_transparency .av-burger-menu-main .av-hamburger-box strong {
    color: var(--enfold-header_replacement_menu_color); 
  }
}

you can try that in quick css:
But maybe that is only nice for wide screens – if there is not enough space it will be not advisable

@media only screen and (min-width:1300px)  {
  #top .avia-content-slider {
    overflow: visible !important;
  }
  #top .avia-content-slider .avia-slideshow-arrows a.prev-slide {
    left: -70px;
  }
  #top .avia-content-slider .avia-slideshow-arrows a.next-slide {
    right: -70px;
  }
  #top .avia-content-slider .avia-slideshow-arrows a.prev-slide:before{
    text-indent: 15px;
  }
  #top .avia-content-slider .avia-slideshow-arrows a.next-slide:before {
    text-indent: -15px;
  }
}

Select an alternative value for 1300px for your installation

on each Enfold Advanced Layout Builder (ALB) Element – you have the chance to save that element as template:

you now can insert this ( completely styled and filled with content ) ALB Element to different pages/posts – on top right of the layout builder window there is a drop-down:

after Insertion ( top or bottom ) you now can drag that element to the wanted position.
___________
by the way: these templates could even be transformed to different installations ( but you had to think of transfering the content used inside these elements by yourself ) :

first tell me if my suggestion from above is correct.
it belongs to the post-navigation left/right arrows? If you have opened a single post – there are left and right arrows to navigate to the other posts.
And now you only want to reach other posts inside the same category ?

And you put that filter to the child-theme functions.php ?

try : maybe the is_fullwidth setting is only missing

function enfold_customization_postnav($settings){
  $settings['is_fullwidth'] = false;
  $settings['skip_output'] = false;
  $settings['same_category'] = true;
  return $settings;
}
add_filter('avf_post_nav_settings','enfold_customization_postnav', 10, 1);

here is the github part showing the long version with comments on that filter:
https://github.com/KriesiMedia/enfold-library/commit/969ff37a8219fee349bb94fcfdbd28fd02d22713

for combo widget i had to look into the code now …
… guess – as mike said no child-theme working solution. – you had to place the widget from the plugin itself.

The most common mistake made when preparing SVG files for fontello upload is not to make the paths compound.

i would follow Mike – and try the plugin. But here is the way i mentioned.
this to your child theme functions.php:

function wp_tinymce_more_buttons( $buttons ) {
	if ( ! in_array( "fontsizeselect", $buttons ) ) { $buttons[] = 'fontsizeselect'; }
	if ( ! in_array( "backcolor", $buttons ) ) { $buttons[] = 'backcolor'; }
	if ( ! in_array( "alignjustify", $buttons ) ) { $buttons[] = 'alignjustify'; }
	if ( ! in_array( "underline", $buttons ) ) { $buttons[] = 'underline'; }
	return $buttons;
}
add_filter( 'mce_buttons_2', 'wp_tinymce_more_buttons', 99999 ); 

// if you choose here  mce_buttons_3 - then it will be in another buttons-row

function add_font_size_to_tinymce( $initArray ){
    $initArray['fontsize_formats'] = "9px 10px 12px 13px 14px 16px 18px 21px 24px 28px 32px 36px";
    return $initArray;
}
add_filter( 'tiny_mce_before_init', 'add_font_size_to_tinymce' );

try this in your child-theme functions.php:

function ava_exclude_current_post($query) {
  if (is_singular('post') || is_singular('portfolio') ) {  
    $exclude = avia_get_the_ID();
    $query->set( 'post__not_in', array($exclude) );
  }
}
add_action('pre_get_posts', 'ava_exclude_current_post');

you can use as if clause instead:

  if(is_single()){  …

Your setting only makes sense – if the header stays fixed on responsive case. So on the css above it is set to fixed
the 116px is the standard setting of header hight on setting: large

– if you got this “Reading Progress Bar” active it might be a decision to show it on scroll or not – that makes only a difference in translate-y amount.

see: https://enfold.webers-webdesign.de/

now there must be a setting for burger active …
best would be to show the header always ( because burger-menu and hamburger icon are part of the header )

.responsive.av-burger-overlay-active #header {
  position: fixed !important;
}

PS: you see on the css above that the media queries rules are the same (in this case) – but it might be different on your settings – f.e. to have a smaller header hight on screens less than 768px.