Yes, it may be the case when, for example, you are calling 3rd party plugins in content or using specific page layouts. You have a global option to force validation of SEO criteria. When it's activated you can click on the criteria icon to validate it. 

Because it may help in some cases for SEO, mainly for multilingual websites, it helps search engines to validate the page language. Some automatic SEO tools may use it too.

Yes with most of them, at least with all plugins and themes that are using custom post type such as WooCommerce. Plus, Yoast SEO and All in one SEO content can also be imported.

All your SEO content is properly stored in the database and automatic SEO checks are also cached or run every 72 hours (unless you force the process)

A tool is available from the bulk image editor, an edit meta button will load all images instance. So you'll be able to define SEO info for each image instance.

Not at all, the images will be replaced/renamed in all your WordPress content so you won't have any 404 errors. Google hates 404 errors :)

If you resize images in editor using visual guides (handles), the image will still be loaded in full size but with bad visual quality.
WP Meta SEO can resize/replace images in all your content, means that if an image is used several times in several articles, you can optimize all image instances.

Now you can replace and change in the dedicated place the good size of the image without changing the original one. Your website will be faster!