Interaction between On Premises AD, Office 365, Directory Sync and Exchange Management

I've been tasked with getting Office 365 synced with our on premises Active Directory. I have some questions about how the Exchange management side of things is going to work, if some of you are familiar with how this all works...

First though, some important configuration information:

• We have AD at the Server 2012 R2 functional level but the directory schema has never been extended with Exchange attributes and we've never had an on premises Exchange service.
• Up until now we have been provisioning accounts separately in both on premises AD and Office 365. The account user principal names match up 1:1.
• We do have distribution and security groups in Office 365 that don't exist in on premises AD.

I have some prior experience with Exchange management through PowerShell. The questions I know to ask so far are:

1. If I ensure that I populate the proxyAddresses attributes on the user accounts in AD with the matching email addresses in Office 365, the directory sync process will correctly and non destructively link the Office 365 account with the AD account, even though I haven't done a sync before, right?
2. Anything special I need to do for the preexisting groups in Office 365? Will they get deleted? Can I migrate them to on premises AD?
3. What happens with the schema for our local directory? Do we need to do anything?
4. What happens to management of Exchange specific attributes? Things like send as permissions, mailbox permissions and attributes that are normally part of the Exchange schema extension? Where are they stored? Do I have limited management? How do I (or can I) create distribution groups in on premises AD that are fully usable in Office 365?
5. What happens if our AD schema is extended later?

Has anyone had these questions answered before?

Thanks