2. Oversized select element in Gutenberg editor (https://prnt.sc/22s0wyj).
This issue will be fixed in the next theme update.

4. Metabox positon.
We will consider your suggestions as a feature request.

8. Gutenberg editor backend.
In the next update, we will add the ability to disable theme styles on Gutenberg editor window.

Like an alternative, for now, you can overwrite editor width by adding the following code to the child theme function.php file.

add_action('admin_head', 'wd_admin_fix');

function wd_admin_fix() {
  echo '<style>body div.block-editor .editor-styles-wrapper .wp-block:not([data-align="full"]) {
	max-width: 840px;
}</style>';
}

9. Article sidebar.

We have checked “SIDEBAR OPTIONS” on your “La Couleur Corail” article, and they are working correctly.

Right sidebar enabled:
Backend (https://prnt.sc/22s10ni)
Frontend (https://prnt.sc/22s0jgs)

Sidebar disabled:
Backend (https://prnt.sc/22s11n6)
Frontend (https://prnt.sc/22s0iwb)

Kind Regards