Well to post an update!!
I decided to have another go at X11 forwarding and realised it was actually the OS X client needing updating than any of the OMV config . Boo. But what it did let me get my head around is a) SpiderOak support seem to be overly cryptic and b) There is actually no need to set Sync folders from the CLI as long as you have a GUI on another system ( i.e. the one you want to 'Sync' with).
In order to use any folder ( other than the Hive folder which I have disabled) to Sync it must first be selected as a 'Backup' folder... This we know can be set via CLI. Once this is done, you can use the GUI on your client ( in my case MBP) to select a current backup folder on your local computer, and a backup folder on a remote computer and voila!! This designates the two folders as a sync and can be started via cli or via scheduling.
So basically SpiderOak support should have explained though you cannot select a folder to sync via CLI, all you need to do is have it set as a backup folder and then select via another GUI...
I'm pro SpiderOak again... for the timing being!! haha