If it's really Jack's dad in the cabin, then it's his corpse. The producers have long said that once you're dead on the show, you're dead.

BUT is Jacob trying to re-animate Christian Shepherd's body -- or even somehow inhabit it? I got the impression in TMBTC that Jacob was more of a disembodied entity trapped through some voodoo in the cabin.

Did he get Locke to find Christian's body and bring it to the cabin? Is this how he wanted Locke to "HELP ME?"

It was not the real, corporial Christian Shepard. We all know that Smokey takes the form of dead people, so that's probably what that is.