From the opinion piece:
Last year, I pointed out how many big publishers came crawlin’ back to Steam after trying their own things: EA, Activision, Microsoft. This year, for the first time ever, two Blizzard games released on Steam: Overwatch and Diablo 4.
Ah, it’s unfortunately not in the contract. They do it (See for example this answer where another family member got the account after their little brother died), but it looks like it’s not a guarantee.
As for the big red button at Valve, I can only find snippets of information. The only “official” one: https://imgur.com/4sa1Ln6 Most information is just from interviews or private messages, claiming there is an inner senior circle at Valve and they do have a contingency plan.