Arbitration Mailboxes in a hybrid environment

Hi,

Planning to put in a couple of servers purely for hybrid use and move all the mailboxes in our main site to Office 365 then remove the old servers, moving the arbitration mailboxes to one of the hybrids, plus ADFS, AAD and so on. Subsidiary sites will stay on on-premise Exchange.

However I had a thought about availability of the arbitration mailboxes. My questions are:-

1. If arbitration mailboxes such as the Federation mailbox for example are down for any reason what problems could this cause for hybrid working? Would free/busy lookups etc. fail?

2. If this would present a problem would the best idea be to put in a 1 database DAG for the hybrid servers and put the arbitration mailboxes in that database?

Thanks

NT