Thanks for your post!
The easiest solution to this would be the iRig Keys I/O series (
https://www.ikmultimedia.com/irigkeysio). This series will offer a headphone output, line level outputs, as well as MIDI capabilities. This series of keys will also offer a PSU (PSU3a) for charging your iOS device durning use.
For your current set up, you can use the iPhone Lightning Dock to connect iRig. This dock has a headphone jack that you may use to monitor your input.
For more information, check out:
http://www.apple.com/shop/product/MGRM2 ... dock-whiteTo charge your iOS device in this case, the iRig PowerBridge would be the best solution.
https://www.ikmultimedia.com/irigpowerbridge/