Ok wow I wrote out the paths I can think about.
I've grouped by meta-scenario and then by actors and mini-scenarios within that. I notice that marking accounts (in the theft meta-scenario) will likely introduce new paths we want for the 'happy' path

0. happy path

Protagonist

have secret
  └─> shards secret 
      └─> ... recovers secret (v1 done)

Custodian

receive a shard
  ├─> gets gets request to return shard
  │   └─> returns shard (v1 done)
  └─> returns shard