AT:

• existing business key from parent process is passed on in call activity with extension element as introduced in CAM-1086
• called process can be retrieved by business key using history service: HistoricProcessInstanceQuery#processInstanceBusinessKey

please find attached your twitter demo that I modified with six test cases.
(please note it won't work in browser since starting from task list with business key is not implemented)

I tested with history level "audit" and "full".
I also tried to set business key instead by setting it in execution listener on start event of called process, but could not find it with history service either.