I use a Motorola Razr Bionic via Bluetooth. If I play Pandora I can skip the current tune with iDrive - but not go backward, of course, since it is streamed audio. I can, however, both skip and backup a tune via iDrive in the (Android) phone's My Music collection. While playing a stream like Pandora or Ustream the tune name on the vehicle display is not correct; it is usually the last one played from My Music.
I also find that I can play both Pandora (for music) and Ustream (for monitoring marine radio) simultaneously; in that case, the iDrive controls Pandora - most times. It might be that the order of app execution determines which is controlled. I'll play some more with this, but I believe you should be able to control your iPhone - but, if you cannot, the phone is likely at fault.
'97 528it, '99 X5, '03 X5, '04 528i, '05 X5, '05 330ci, '07 X5, '07 X3, '13 X3, '15 328xi