This would give you protection both against the amnesia route (where you fall unconscious, lose your memory but are totally fine afterwards) and the route where you're unable to manage your secrets at all (eg stroke resulting in longterm failure to maintain memories or make decisions).

You'd still, for the total lose route, need a replacement actor (someone acting on your behalf) to assemble and receive the key, and be the keyholder moving forward - and you would likely need to leave instructions with the flock of people having pieces of the key on how to select or confirm your future keyholder.

Not necessarily. Bank safe-deposit boxes are a secure place to keep secrets. To guard against rogue bank employees, encrypt the stored secrets and keep the key at home on a sticky note. If you ever hit your head and forget all your secrets, just present your ID to the bank teller, pull the secrets out of the vault, and decrypt them with the key on the sticky note.