hybrid exchange (2010) migrate to new exchange 2016 server in new (trusted) domain

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Scenario:
ExchangeServer2010 (hybrid) is a member of DomainA which has a 2way trust with DomainB.

ExchangeServer2016 (which is not yet in production) is a member of DomainB.

Objects of DomainA are synced by Azure AD Connect with Office365.

ExchangeServer2010 hosts on-premise mailboxes (both user and shared) and a set of remote-mailboxes (both user and shared). 
All our email from outside our organisation flows into our ExchangeServer2010 and decides if recipient is on-premise or in Exchange Online and sends it accordingly. so far so good.


What we want:

We want to decomission ExchangeServer2010 in DomainA and migrate all on-premise mailboxes  (and their user-objects) to ExchangeServe2016 in DomainB.
But also, we want to keep all user-objects with remote-mailboxes (both users and shared) in DomainA and keep them synced by Azure Ad Connect synced with Office365 and manage their mailboxes in Office365.

We want our future mail-flow like this:
Have mail from outside our organisation flow to ExchangeServer2016 and decide if recipient exists on-premise and if not check with and/or forward to Exchange Online.

We are not sure how we can 'divorce' our remote-mail users from ExchangeServer2010 or if this is possible.

And we dont know how the last part in our future mailflow (decide if recipient exists on-premise and if not check with and/or forward to Exchange Online)  is to be configured on ExchangeServe2016.

Thank you to anyone who can shed some light on our situation!

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