I have been able to upload/download tracks & waypoints on the move from/to the Fenix to/from the cloud using BaseCamp mobile 2.0 iOS app.
But the procedure if far from "intuitive" ...
1) get online on the iOS device (I have to use 3G/WiFi tethering from my Android smartphone because I have the wifi only iPad mini)
2) launch your BaseCamp mobile app and log into your myGarmin cloud account.
3) pair/link the fenix and your iOS BT40 device as usual.
4) once you see your fenix resident tracks & waypoints listed on the basecamp mobile's "device" tab then do "edit" the one that you want to upload but the, instead of renaming it, just tap "COPY" !... No questions are asked (copy to where? with what name? etc) nor any confirmation of transfer success is given but... then you go to your "My colecction" tab and that item you just "COPY"ed is there allready !
To load an item (track/waypoint) from your myGarmin cloud collection into the fenix you just have to "EDIT"+"COPY" that item from the "My collection" tab and it will show up on fenix "My device" tab.
At least now I have a method to clear up (or backup) the log memory of my fenix before I return home to a PC with full BaseCamp sofware. Too bad an online connection is still required to achive this, I would much prefer to be able to store/copy the tracks localy on the iPad. I can do that using the OTG USB cable of my smartphone but I find myself mostly leaving the fenix cable/craddle at home, so this 100% wireless method is greatly welcomed.
Edit: screen captures of the cloud -> fenix procedure, red dots on where to tap