Plex recently announced a new service called Plex Cloud which is currently in Invite-only Beta, but you could try registering for it to see if you can get a Beta invite.
Basically it will allow you to run a Plex media server in the cloud and stream the video off your ACD storage. (They also recently announced a partnership with other providers besides ACD as well). That way you don't need to run a local Plex media server at all.
How does odrive play into this? We make it a lot easier to get your files into your ACD account. This techcrunch article mentions users having problems getting their files up to ACD.
So basically, the way I see it working is like this:
1. Use Plex Cloud to have your Plex media server running in the cloud instead of your local machine. This Plex Cloud server will be able to play your files off ACD.
2. Use odrive to upload your files to ACD and to manage it more easily (since all you have to do is use Finder / Windows Explorer). The odrive sync app will intelligently try to upload the files for you and should circumvent the upload problems that are causing other users to fail.
3. Since your Plex media server is no longer on your local machine, you can unsync files locally just fine without affecting your ability to play stuff off the Plex Cloud server instance.
If you manage to get this up and running, we'd love to hear about how it goes!