You have an Exchange Server 2010 organization.
You plan to upgrade to Exchange Server 2016.
You have two active directory sites configured as shown in the following table
Hot Area:
Answer: 
You have two active directory sites configured as shown…
You have an Exchange Server 2010 organization.
You plan to upgrade to Exchange Server 2016.
You have two active directory sites configured as shown in the following table
Hot Area:
yes /no/no
Looking at the namespace configuration here’s what I understand:
1) CAS02 will not proxy/accept connections for OWA because there is no virtual directory configured at all
2) All users access OWA by using https://mail.contoso.com/owa since it is the only namespace that’s accessible from the internet
3) If you try to access OWA by using https://mail.brussels.contoso.com/owa it won’t work because it is not internet facing
When USER1 connects to https://mail.contoso.com/owa EX03 redirects the connection to the site that holds his mailbox, through CAS01
When USER2 connects to https://mail.contoso.com/owa the same thing happens, except CAS02 is not configured, so I suppose it is redirected to another CAS server in the same site (CAS01)
When USER3 connects to https://mail.contoso.com/owa there’s no need for redirection as his mailbox already resides in the 2016 environment to which the namespace is pointing.
So, I would go for yes/no/no
You seem right, although User1 will be unable to connect after redirection to CAS01, as the site has no internet connection.
User2 will not be redirected to CAS02, as there is no VD as you mentioned. And User3 will not be proxied or redirected to CAS02, as it will be connected to the same site, and just be proxied to it’s mailbox.