HACS is as official as it’s going to get. Any more blurs the line of supported. And the API is already fully documented:
Documentation - Home Assistant (home-assistant.io)
and
Home Assistant Developer Docs | Home Assistant Developer Docs (home-assistant.io)
This is a separate ask - also NodeRed exists
So like how there are 2400 integrations in core. Each of which went through a code review/moderation process before it was added to core and continues to go through a code review/moderation process for every code change.
See NathunCu’s links above. There’s already an developers.home-assistant.io site dedicated to giving developers the info they need to make and maintain integrations.
Not really sure what a “developers console” is but if you’re a developer working on an integration you can test your new version easily. If you take a look at hte developer docs you’ll see it shows you how to set up devcontainer in your repo so you can have a one click task to run HA locally with a local build of HA including your pending changes. You can also test on any existing system by overriding the code of a particular integration using the custom_components folder. All of this is covered in the developer docs.
Sure. Sounds like a cool idea. Feel free to make and contribute them if you have the time.