Adding extra product info

Have you seen this plugin:
https://wordpress.org/plugins/custom-post-widget/

You can write your extra info as a content block, and the plugin gives you the shortcode to be able to insert it where you please.

If you later update the content block, the update shows everywhere the shortcode has been used.

Yes.

You can show the description before the tabs with some code:

add_action( 'woocommerce_after_single_product_summary', 'show_product_description',  5 );
function show_product_description() {
  global $post;
  the_content();
}

You’ll still see the description in description tab, so to remove that, unset the description tab as explained here:
https://docs.woocommerce.com/document/editing-product-data-tabs/

But if you want some text before the tabs, and some in the description tab, then it gets complicated. You’ll need to setup a separate meta box to capture the additional text and then show it using the add_action() similar to above. Creating an additional metabox is beyond the scope of a forum answer.

I don’t know of a plugin that will do this but someone might.

I have assumed your product page template is using the WooCommerce standard arrangement. Some themes have their own templates.