Reference: https://one.google.com/faq/storage
“What happens to my files if I go over my storage limit?
If you go over your storage limit, you won't lose anything. However, you won’t be able to store new files in Google Drive or store new photos or videos in Google Photos, and you may not be able to receive emails with your Gmail address.
Need more room? You can learn more about clearing up space or upgrade to Google One to get more storage.”
>No. You can’t decide how much storage each person in your family group gets. It’s worth noting that everyone in your family group already has 15 GB of storage at no charge with their Google Account. They only start using the shared storage from Google One after they use up their 15 GB.
https://one.google.com/faq/plan-sharing
It doesn't sound like it. I think you can only have 1 plan and may cross out.
SOLVED:
1)
https://support.google.com/youtube/answer/7507349#requirements is more informative than https://one.google.com/support !!! Lists new/open cases much sooner. Still doesn't include chat logs, which I remember being told would be - and remember were - accessible at the latter URL or something close.
2)It seems like I misremembered what was included in Google One - It was Google Play (since discontinued) that included YouTube Premium and YouTube Music? Hard to figure out. Google One support claims that no features of Google One have been removed - but of course that's obviously false - Google Play is gone! (...though YouTube Music is close to a full replacement.)