Call API on post save/update and show the result in admin area

You can’t do that with PHP. You must use Javascript-React to alter/enhance block editor behaviour. You’ll have to use subscribe function, for subscribing to actions, catch isSavingPost action there (there is also isAutoSavingPost for autosaves) and dispatch a Notice in the editor with appropriate action. Familiarity with React and Redux would be of great help.

What Role to assign remote site developer?

“Developer” implies that he will have access to the code – in which case he has carte blanche to do whatever he likes. If you actually mean he just has access to the site admin – and I assume this is multisite as you mentioned being a Super Admin – then a site Admin cannot … Read more

Layout Settings in the new Gutenberg editor

For a very basic implementation of adding a text input to add body classes(would also work for adding a second field for post classes if necessary) to pages/posts, you could do the following: Register a custom field with register_post_meta – I’ll use an empty string for the first param, post_type so it applies to all. … Read more