I have a different My Cloud device, the DL2100 NAS, nevertheless, they are similar enough to respond to your questions. This is what I can and cannot do with my iTunes My Cloud app and my DL2100.
First of all, the My Cloud app has some major issues with music, and I and others have brought them up to WD with no resolutions so far. My Cloud app cannot play from playlists. If I want to play from playlists, I use my Android Kindle tablet (or WDTV). There is an app called Avia, that can access the My Cloud devices I have (DL2100 and My Passport Wireless as a MEDIA SERVER and since both these devices have built-in Twonky server, they can serve up playlists using the Avia app, so I call them up on the Kindle and even send output to Bluetooth devices. I have not found an iOS app that can do this. yet The My Cloud app is great as a video player, though.
The other issue with My Cloud app is the iOS device has a feature called Auto-Lock, and My Cloud cannot handle this right. If auto-lock setting is shorter than a track’s time length, in most cases My Cloud app cannot advance to the next track, and I have to advance the track manually in app. The Android version of the app does not have this issue.
My iTunes library on my NAS and MPW are copies of the folder on my PC; iTunes Media. I can access it from internet with My Cloud app, but I can also access it from an app for my Asus router called AiCloud and since I do not have issue of locking up as I do with My Cloud app, I prefer using it for listening to music .
About download, I had never tried it until today and it works fine. I keep no music or videos on my iPhone or iPad, so I tried the Download feature of My Cloud and downloaded one song and one album from my NAS to both using app. They showed up in Downloads section, I played it, and just as importantly I could also delete it. This is a nice feature I may use just to keep “emergency music” on devices for when streaming is impossible due to no internet.
No artwork shows up, and I really don’t recall ever seeing artwork in My Cloud app, although other apps show art if available. Accessing music from my WDTV does allow artwork (if there) to show on TV.
It appears that My Cloud app needs some serious work.