Decommission On-Premise Exchange 2013 after move to Office 365

Environment: 

1x On-premise Exchange 2013 Server with CAS role

1x On-premise Exchange 2013 Server with Mailbox Role

DIRSYNC

All users migrated to Exchange Online

Questions:

1. In removing the Hybrid Configuration, there's a step that requires the On-premise EAC to be used. However when I open the EAC in the On-premise CAS server I'm redirected to the Exchange Online OWA page. Question is how to open the EAC in on-premise when all users have been migrated to Office 365? Is there a need to off-board the admin mailbox in order to run the EAC?

2. I need to decommission the On-premise Exchange 2013 Mailbox with the intention of reclaiming the storage space used by the databases. The on-premise Exchange 2013 CAS server will be used to manage the Exchange attributes and will not be decommissioned. Can you point me to a documentation on how to properly remove the Exchange 2013 Mailbox role?

Thanks.