Questions
1) Exactly how did Father Luzzi find Abby? He said by "following the signs", like what?
2) How come NO ONE realizes that Father Luzzi isn't aging except 1 priest?
3) What happened to Father Luzzi at the end?
Show me the holes!
1) We saw him reporting to the cardinals, who in turn reported to the Pope. So the Vatican has been monitoring them but he puts their fears to rest. Maybe this pattern starts up from time to time and they check it out.
So as he verifies them and realizes that it appears to be coming true, he ends up in Abby's vicinity. I haven't seen it for awhile but I guess he finds her tenant (David Bannon), follows him, and sees her. Or at least, Lucci realizes she must be nearby.
2) I don't know; possibly changing aliases. The guy that recognizes him, confronted with the notion that it's "impossible" because of the elapsed time, seems to question his own thoughts. One thing, though, is that before getting that high up (to indirectly have the Pope's ear) I would think a background check or something would be required. If he's really Cartophilus, then maybe he murdered a priest and assumed his identity, that sort of thing.
3) I don't know.
Re: 3):
I would guess that Cartaphilius - since he lost his "End Times" opportunity, was cast back into the "Hell" of his earthly role of wanderer until Jesus returns at some remote future date. I'm presuming that Abby's sacrifice permitted the human race to barely squeek by, with God, who activates a rather minimal grace-period, declaring: "Okay, for now I'm suspending judgment and withdrawing my Son back into Heaven with me. But be warned: there WILL be a world judgment sooner or later - sooner if people do not emulate Abby's example of sacrificing love. Then when my Son returns he will separate the sheep from the goats and the entire world will be judged" ... or words to that effect.
So, the horribly disappointed and emotionally shattered Cartaphilius - since he cannot die until Christ truly and with finality returns to judge the world - takes up his "cross" of Guilty Wanderer through the human ages... thereby almost becoming a tragic figure in his own right.