The Grail, The Knight and Immortality.
I've read a few post about those things listed above, and I think a lot of people don't understand them. Here's my take on them.
Drinking from the Grail once does NOT make you immortal. We find out in KOCS that Henry has died. It gives you life, but not infinite life. For example Indy might have been 10 years away from having a massive heart attack. Drinking from the Grail may have pushed that back another 20 years as well as heal up other damage he's done to his body over the years. He was in his 60's for Crystal Skull but fighting as well as he did in his 30's. Henry was minutes away from dying of a gun shot wound, but instead he lived at least another decade.
As to why the Knight was able to live 700 years, he drank from Grail everyday. So he kept giving himself a little more life everyday, so that he could fulfill his duty, until his replacement arrived. And while the Grail could keep him alive and somewhat mobile, there's only so much a body can handle after 700 years, even with the Holy Grail. He might have been able to help himself more if he spent more time staying in shape.
Finally in regards to crossing the great seal. The Knights says "The Grail cannot cross the great seal. That is the limit, and the price of immortality." People who drink from the Grail can leave the cave whenever they want. The Knight himself could have left at any time. However the Grail itself is what is not allowed to cross the great seal. So if you want live forever and continue drinking from the Grail, you have to stay in the cave with it like the Knight did. That's what he meant by the limits of immortality. You can live forever, but only in this cave.
As far as how the Knight felt about the cave collapsing and his imminent death, most likely he was at peace with it, and ready for it. His duty had been fulfilled. He guarded the Grail until he was defeated by a successor (he surrendered his sword to Indy), and now it was going to be buried deep beneath the cave where it would never be found.