Are you sure the file is renamed and you update the namespace?

Did you rerun php bin/magento setup:upgrade && php bin/magento setup:di:compile?

attr: {href: product_url} should remove the link for every product. Take a look at app/code/Magento/Checkout/view/frontend/web/js/view/cart-ite‌​m-renderer.js and app/code/Magento/Checkout/view/frontend/layout/checkout_cart‌​_sidebar_item_render‌​ers.xml

Take a look at /app/code/Magento/Checkout/view/frontend/web/template/minica‌​rt/item/default.html or vendor/magento/module-checkout/view/frontend/web/template/mi‌​nicart/item/default.‌​html if installed using composer

Adding tracking scripts and testing your success page will become unmanageable over time. You should consider using a third-party extension. eg github.com/magepal/magento2-google-tag-manager and github.com/magepal/magento2-preview-checkout-success-page

Anywhere on the page, assuming your gtag/gtm tags are already loaded.

Instead of hardcoding this into your success.phtml template, you may want to consider using a GTM extension. eg github.com/magepal/magento2-google-tag-manager

Take a look at stackoverflow.com/questions/280014/…

Take a look at magento.stackexchange.com/questions/110164/…

To increase the likelihood of your emails getting delivered to customers' inboxes, you could consider using an SMTP extension. eg github.com/magepal/magento2-gmail-smtp-app

"localhost" should be the server itself. Is there an FTP server running on that server? Can you anonymously FTP into that server? From your command line try ftp localhost (you may need to use the webserver user since the PHP script is running as that user)... You may need to use ftp_login

You will have to use a session to temporarily store and then pass it to private content.

More than likely this is a FPC (full page cache) issue. To do something like this you would need to use the "Private content" and set various sessions. developer.adobe.com/commerce/php/development/cache/page/…