I don't see how the ghost could be used in the LTD.
For some reason the programmers connected a random NPC to the ghost however; and talking to that NPC will remove the ghost from the church. Me no likey. So I have to figure out which NPC it is; I may be getting some help with that.
All you have to do is visit the church again after you left it on disc 1. Without cheats you can't do this until late on disc 2, but the ghost actually exists in the church even when Aeris is in your party. The ghost will just be tending the flowers, next to two kids, and trying to speak to the ghost will give no dialogue. If you pass a certain safe point in the church the ghost fades away permanently. (The same if you speak to this random NPC)
Not always, but like I said once you leave Midgar for the first time the ghost exists there. So with cheats, giving you the Sector 5 Key item, you can go see it. However without cheats the earliest point will be right after Diamond Weapon attacks, and you can dig it up in Bone Village.
Yes, you should be able to see it indefinitely as long as you don't make the ghost fade away.
I'll try to inform as many as possible whenever this inconvenient part of the "ghost mechanics" is figured out.
I might also mention to the interested that a new "Super W-Item Glitch" has been uncovered. It is a way to duplicate (some) items which are not selectable on the item list during battle.