You can cross off #4, there is a way to do this in the official app. While navigating to a cache, tap the flag icon in the upper right hand corner, next to the compass button. Input your new coordinates and hit "Done". A flag will show up on the map at the new waypoint. Tap on it, select "Set as Target". Off you go!
I send you wholehearted thanks for the above - I was tearing my hair out not knowing how to input co-ords not only in the IPhone5 GeoApp, but also in Iphone MapsApp. Now I can forget trying with the mapsApp. Geo App is my only Geocache hunting tool, and I had almost decided to just ignore offset caches, or puzzles, anything that required manual co-ords input. Thanks to the valuable info you have provided, this side of Geocaching can now be open to me:-)