autotutorial

$_SERVER['DOCUMENT_ROOT'] = '/www/home/user/public_html';
I specified the root of the document to be as exhaustive as possible.
You need to enable the rewrite engine and add the path to where your htaccess file resides.

Good for root /www/home/user/public_html:

# BEGIN WordPress

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

# END WordPress

RewriteBase is for root /

You must copy your index.php file into root folder and edit
Original:
require( dirname( __FILE__ ) . '/wp-blog-header.php' );
/www/home/user/public_html:
require( dirname( __FILE__ ) . '/new_path/wp-blog-header.php' );
Now it /www/home/user/public_html/new_path
If you use subfolder for WordPress.

Unistalling

yes, I think a plugin other than yoast Seo adds html markup to everything.

Example of xml and declaratiion, well formed.

<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="//yourdomain/wp-content/plugins/wordpress-seo/css/main-sitemap.xsl"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
	<sitemap>
		<loc>https://yourdomain/post-sitemap.xml</loc>
	</sitemap>
	<sitemap>
		<loc>https://yourdomain/page-sitemap.xml</loc>
		<lastmod>2019-12-09T12:38:39+00:00</lastmod>
	</sitemap>
	<sitemap>
		<loc>https://yourdomain/oceanwp_library-sitemap.xml</loc>
		<lastmod>2019-11-19T09:31:26+00:00</lastmod>
	</sitemap>
	<sitemap>
		<loc>https://yourdomain/ocean_portfolio-sitemap.xml</loc>
	</sitemap>
	<sitemap>
		<loc>https://yourdomain/author-sitemap.xml</loc>
		<lastmod>2019-12-08T18:23:28+00:00</lastmod>
	</sitemap>
</sitemapindex>
<!-- XML Sitemap generated by Yoast SEO -->

To catch the error your server needs to enable errors, consult your manual this is an example when php was installed via apache module, now it is set via php.ini and obviously the syntax changes a bit but those are the variables necessary.

# enable PHP error logging
php_flag  log_errors on
php_value error_log  /home/path/public_html/domain/PHP_errors.log

# supress php errors
php_flag display_startup_errors off
php_flag display_errors off
php_flag html_errors off

https://perishablepress.com/how-to-enable-php-error-logging-via-htaccess/

In your wp-config.php write this syntax.

define('WP_DEBUG',true);
define('WP_DEBUG_DISPLAY',false);
define('WP_DEBUG_LOG',true);
// If error is true, it create wp-content/debug.log file

Debug JavaScript , CSS and other https://developers.google.com/web/tools/chrome-devtools.

You currently have two problems. 1) Each non-existent path is transformed into a query string (either a page or other) 2) Currently the browser (without Chrome private mode) cache makes a redirect to the homepage on any article. If you create a redirect loop in the htaccess file, do you get stuck?

Redirect 301 / http://yourdomain.com/

http://www.cabinet-hypnose-la-rochelle.fr/fantastic_url/ does not generate an error 404 instead turns it into a query string, do you use plugins to handle 404 errors?

here are the terms to remember.
Default: "Ugly" http://example.com/?p=2
PATHINFO: "Almost Pretty"
http://example.com/index.php/yyyy/mm/dd/post-name/
mod_rewrite: "Pretty Permalinks" http://example.com/yyyy/mm/dd/post-name/

to use a structure without the date, you have to choose custom size.
Permalink https://wordpress.org/support/article/using-permalinks/
Ningx https://nginxlibrary.com/wordpress-permalinks/

the old path is not forwarded to the new one, you have to use a plugin for this.

Note: you cannot use a new route for many date paths or otherwise you will be penalized for the seo, you can an old route in a new one instead.

can you create a test environment, copying the file and database, then with one plugin (search and replace) search old domain and replace with new domaim, do you also have permalink problems on the test site? if you disable all siteorigin plugins (even widgets) the problem disappears?

https://en.wikipedia.org/wiki/Well-formed_document
You can’t mix html markup with xml, you need to find which plugin adds html markup to an xml file.
means an xml file must not start with html markup and optionally can have an xml declaration on the first line.