Assumptions?
Grace has shared their .ssb/secret with 5 friends.
At some later point Grace has become incapacitated.
Some number of the 5 friends is aware that Grace has become incapacitated.
What next?
Without getting into the technical details we can make some social guesses and scenarios which may happen. We can guess that word will get out that Grace is incapacitated. There may well be some type of announcement about Grace being out of action. Let us assume then that some high percentage of the shardholders are aware Grace is out of action. Socially it would make sense for those who have participated in receiving a shard from Grace to know - hey, I probably need to do something now. Quite likely in the event no other course of action has been laid out there would be human gossip amongst the social network about what's up with grace and also asking eachother if anyone else was a shard holder and if anyone knows what happens next.
In Cryptoasset Inheritance Planning: A Simple Guide for Owners Pamela really drives home that it is not enough to just get the private keys to the end recps (heirs?). Will they know what to do with it? Will they have the concepts, the skills, the context?
What is suggested is to assign Executors and Technical Aids. Within this context we should be thinking not just about getting key backed up, but setting up social circles and social fabric which will be able to make use of the thing being passed on.
Perhaps Executors get given a private message which is readable after decryption which stipulates a course of action in the even of coconutting. Perhaps there are hash links to people Grace trusts to offer technical expertise. etc - you see where I am coming from here...