can I link the SQ7, AR24x12 and AR84 via a network switch and use them separately
No. You need to connect the AR84 to the Expander socket on the AR2412.
You can keep the AR2412 switched on for using the AR84, you don’t need to use it for I/O.
A direct connection to the AR84 or an AB168, giving you more I/O would be possible too.
And the AB168 can connect to the expander port as well.
Or is that something that the DX Hub would do?
The DX hub connects with DX boxes only, not with AR or AB.