You obviously don't pay attention to detail, do you? Indiana and his father are BOTH immortal, and here's why...
1.) How could the seal stop the power of the grail? The grail had it's power long before the seal was created, as Jesus drank from it hundreds, if not thousands of years before the time of the crusades (I don't know the exact time the crusades took place). Also, the seal might have been able to contain it, but it was destroyed in the cave-in. No more seal, no more restrictions. Also, the seal was only near the entrance of the cave. What if they decided to dig a hole through the back of the cave? The seal isn't a big circle that surrounds the entire cave, so they wouldn't be passing it would they? My point on this is that you can pass the seal and be immortal.
Reply: The stone city collapsed because the seal was crossed, but it wasn't the seal per-say that limited the power of the grail it was the area that the seal was placed on...as a marker so the area wouldn't be crossed by accident.
2.) The knight said that another knight would defeat him in single combat, take his place, and then he would be able to rest. This implies that the knight wanted to die. If he wanted to die, why not simply walk past the seal? But how can you be killed in "single combat" if you are immortal? Indiana NOT taking the knight's place shows that he was wrong about the grail in this situation, and implies that he could be wrong in other situations (ie-the seal).
Because it was his duty to guard the grail and prevent the wrong hands from getting it. Watch the movie again, he talks about being chosen to stay, remember "Two of these brothers walked out of the desert one hundred and fifty years after having found the Grail and began the long journey back to France. But only one of them made it. And before dying of extreme old age, he supposedly imparted his tale to a?to a Franciscan friar, I think."
He also never says that he has to be killed only defeated, and Indy did just that. I presume that the knight would have walked out of the temple and died a natural death.
3.) The room was constructed to hold the grail, long after the grail was created. Why would it's power be limited to a room that it was put in a long time after it was first used?
Says who? It is never said who built the grail temple or when. It could be assumed that God placed the limits on the temple.
4.) Why have an entire sacred order devoted to protecting the grail if it's power were only limited to a single room? Doesn't make much sense, unless of course the grail's power could be used elsewhere.
This is illogical. For instance, Hitler could have continued to return to the grail thus, at least, staying alive for quite some time.
5.) I forgot this reason. If the grails power ends after the seal, why didn't Indiana's father get another hole in his belly where he was shot and died?
The healing power doesn't end, since the healing was already done. The eternal life does, since the eternal life thing was still in progress.