Second BIG point:
Institutional instances -- WITHOUT AGREEMENT FROM EMPLOYEES (and preferably from union representation!) -- would by default preserve the employer/employee power imbalance.
Institutions and journalists share an interest in proving identity. They also obviously have diverging needs and incentives.
Journos/staffers are free agents. Their network must be portable. Their content must also be preserved.
I don't have an answer for this, save that it must be negotiated.
Next ALSO REALLY BIG POINT:
It's one thing to have an institutional instance (whether admined by the institution, or provided as a hosted service a la Google Mail), it's another to:
* be sure that Mastodon is a secure service that meets the institution's eg security, privacy, trust and safety needs. That's a new, big lift!
* have the capacity to act on eg law enforcement requests (especially eg when the instance is open to users other than staffers, like readers)