Up front to answer your question from 2013....
- Yes there is - see below
- No, you wont need to
In your first paragraph you seem to imply that from the eDiscovery Center in SharePoint, you will need to query out separately from SharePoint to Exchange (via EWS). After researching this, I found that you should be able to do a single search within your SharePoint eDiscovery Center if you have the administrative settings correct to bring Exchange into SharePoint's Onines eDiscovery Center.
From Office 365 eDiscovery documentation:
In the eDiscovery Center in Office 365, you can search all Exchange Online mailboxes, SharePoint Online sites, and OneDrive for Business sites in a single eDiscovery search. To search all content sources in the organization, an eDiscovery manager must be assigned the appropriate eDiscovery permissions for each content source.
To allow your Exchange to be queried via the eDiscovery Center you need to set it up as a result source. This is Step 2 of Set Up an eDiscovery Center in SharePoint Online.
To search Exchange Online mailboxes from an eDiscovery Center, you have to configure Search to include Exchange Online as a result source for the eDiscovery Center site collection. For more information, see Manage result sources.
Follow these steps to configure Exchange Online as a result source for the eDiscovery Center that you’re setting up.
Go to the new eDiscovery Center that you created in Step 1. Use the Web site address that you specified; for example, https://contoso.com/sites/eDiscovery.
Go to Settings > Site settings.
On the Site Settings page, under Site Collection Administration, click Search Result Sources.
On the Manage Result Sources page, click New Result Source.
In the General Information section, in the Name box, type Exchange Online and, optionally, type a description.
In the Protocol section, select Exchange.
In the Exchange Source URL section, click the Use AutoDiscovercheckbox.
Also - refer to the FAQ for more deatils on setup, case search flow and typical search scenarios. This is a very deep setup topic to broad to cover here. But YES it can be done, and no you dont need authentication in your search - that is setup as part of the admin of SharePoints eDiscovery Center.
Lastly - there is a link here updated Feb2016 on the latest CSOM-eDiscovery protocol - the html is blank now - but if contains a link at the top to a PDF which has detils of how to do CSOM eDiscovery Center query/holds.
Hope this helps...