Ex2013 full hybrid, no edge server, 3rd party for all filtering and scanning transport routing

Occasional Visitor

Hello, have some general questions for clarification around complex mail transport routing scenarios -


1. Does the HCW overwrite the current send/receive connectors in the on-premises CAS server? What about in Exchange Online?

2. I know there will be send/receive connectors for all internal mail flow directly between Exchange organizations, but the mail-flow from the CAS server to external recipients will flow directly to the internet by default using DNS, unless otherwise configured in a send connector, right?

3. If we're using a 3rd party for filtering and scanning of all messages to and from external recipients, will I need to setup the HCW and let it set it's own values, then go back and configure the send and receive connectors in Exchange Online to bypass filtering through EOP, and then set my external send connector on the CAS server for external recipients only to target the 3rd party service?

Basically, I need all external mail to flow to and from a 3rd party (not in-between Exchange Online and the CAS server for incoming) and I need to bypass the filtering in EOP but still use EOP as the gateway for routing to destinations for all mail flow scenarios except the CAS server sending to external recipients directly through the 3rd party. I know how this works theoretically, but the order in which I execute these changes and if HCW overwrites my current connectors is a little fuzzy to me still.

Don't want to end up halting mail flow...