Exchange 2013 - Creating system mailbox and discovery. Not working?

Exchange 2013 environment - several CAS servers, several mailbox servers in DAG.
Four mailbox databases, each ~1.5TB, a few thousand mailboxes.

We need to be able to run a search across all mailboxes, which I believe is the purpose of the 'discovery mailbox' functionality.  When navigating to 'in-place eDiscovery & hold", we got the error message in PIC 1 (attached).

Walked through the steps in the exchange management shell to recreate the discovery mailbox on one of the four mailbox databases.  PIC 2 (attached) shows the created discovery mailbox (which doesn't show up under Get-Mailbox -arbitration, only when I filter on the recipienttypedetails, which to my understanding is correct).

Still getting the same issue (Pic 1).  So in researching, it looks like we need a system mailbox, which the Get-Mailbox -arbitration is not showing.  PIC 3 (attached) shows the Exchange user accounts that are showing up in AD.

Now my understanding is the only way to create a system mailbox, unlike a discovery mailbox (which is done through the exchange shell), is to run setup /preparead.  

I'm hesitant to do this, as it's imperative I don't risk affecting mailflow or other operation of the Exchange environment to the thousands of users.

Looking for some insight and direction here.  Can I run exchange setup with the /preparead flag?  What will that do other than prepare the AD scheme (recreating the system mailbox accounts)?  Is there any way to do that manually?  Again, want to ensure I don't risk the current 6TB+ and 3k user environment.
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Mystical_IceAuthor Commented:
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