Hello Edwin,
Earth Projects are not kml. Yes, you can export a kml, but that file is not the same as the project. That stated, Earth projects do not have all of kmls bells ans whistles; folders and ground overlays for example. And yes, Voyager stories are kml with features that not available for Earth Projects. (Lacking Features described later)
I also agree that there are some kml, <gx:xxx>, tags that are not currently documented, but most of the quiz voyager story linked in your post is not new kml. In fact, the changes people see is a little HTML, CSS, with a bit of JavaScript; components of a web page. They are more sophisticated then an early 2000 websites, but they get a lot of help from
material design lite. For that voyager story linked in your post, probably the only undocumented kml features are show, hide, and maybe a FlyToOrbit here and there.
These quizzes are not easy to make in kml; or in Earth Projects. While creating the buttons and panel are relatively simple, keeping score requires a huge number of placemarks. For example, an eight question quiz can turn out to be over 1,100 placemarks; if you want to keep score with styled "A" "B" "C" map pins. If you don't want to keep score you can create these "interactive experiences" using custom HTML in Google Projects with much greater ease. Not that it is initially easy, but most of my 9th graders could probably figure it out if guided.
As aforementioned, there are still a number of kml features, like folders, time, ground overlays, screen overlays, import from sheets/.csv etc. that I wish Earth Projects could better support. Here are links to two kml stories that are not possible to do well using Earth Projects. I, like you state, share the kml from drive; hoping the user knows how to import kml.
Then panels are easy...the # of points/polygons, creating <style>ing for imported features, not messing up the Table of Contents, and the use of ground overlays are what is problematic. In addition to leaving feedback in these forums, I always try to use the feedback option within the menu of he web app. I am pretty sure the team has heard better kml support; ingesting kml into Earth Projects, but it always helps to have more voices:) I also share your feelings about making all these option, features, collections, more robust at the same time creating an easy user experience; allowing the greatest number of people to explore, create, and share.
Lastly, I believe the enable import kml is no longer an issue. I want to say that it is enable on by default with the last update.
Sorry this got a little long,
Josh
*There might be some issues with sharing Earth Projects if your GSuite Edu account does not allow sharing outside from outside domain*