It sounds like the problem is in the derailleur itself, not necessarily the cables. If you can't get the derailleur to move by pulling the cable, then maybe the der. is stuck/tight. Will the derailleur move just by pushing/pulling the der. body directly (not using the shifter or touching the cable)? A front der. should be sort of hard to move in your hand because there's a strong spring in it, but it should move. You can carefully pull on the der. cage plates a bit. (Be careful to not bend the der. cage.) If it doesn't move, try putting a penetrating lube (I like Tri-Flow) on all four main pivot points. Let it sit for a little while. If the thing still doesn't want to budge, you may have some rust in there. If it is very rusted, a cheap replacement der. might be the way to go.