Looking at Ediscovery platform over and above the enterpirse vault
And wanting to know without archiving a mailbox and enabling journalling
Can the ediscovery platform search through the live mailbox on-premise or in office365
Yes, with eDiscovery Platform it's possible to search against the mailboxes on-prem & O365 without first archiving into Enterprise Vault or another product.
This is the guide for setting it up, this should give you some idea how it works: Identification & Collections Guide
This shows you what is currently supported in terms of collection sources: Compatibility Matrix
ediscovery platform can definitely stand alone and integrates with O365, Exchange, and many other sources. there are major advantages to having email, files, etc, archived in EV but it's not a requirement.