As far as Irina Spalko goes, she didn't ever do anything onscreen that was horrible enough for her to be vaporized by a CGI alien. Her objective was to make the rest of the world Soviet, yes, but there's nothing in this movie to suggest that Soviets were as bad as - or worse than - the Nazis (which they were).
I don't think that she was vaporized so much as taken with them.
The thing is, they gave her exactly what she asked for, but she couldn't handle it. It wasn't malicious. It was just be careful what you wish for.
I might buy the fridge protecting him from the radiation or thermal pulse, but not from the enormous blast itself and the shockwave of debris hitting him at incredible speeds, which surely would have obliterated the fridge right along with everything else. Not to mention the hard crash landing later on...
Yeah, it really should have been farther away from the bomb. At close range it would have been melted and obliterated. At longer range it would have had a good chance of surviving.
They talked about how his (The Knight's) brother died of extreme old age. This was because he crossed over the seal and the power of the Grail no longer kept him immortal.
Nothing in the film says that you need to keep drinking from the grail to remain immortal.
The knight got old and weak inside the temple. So based on that, the power isn't limited to the temple. So why did he get old? A) immortality is not possible with the grail, B) you need to get your frequent dosage of holy grail water to remain the same age forever, but he didn't take it.
Also, Henry's wound remained healed after he left the temple. No reversal of holy effects either.
The knight didn't just become old and weak. He became less substantial. By the point they found him he was less a living person and more a corporeal ghost chained to the world by the grail and his oaths.
He's certainly immortal, but it isn't what it's cracked up to be and by that point he isn't just physically unable to continue his vigil, he's spiritually exhausted and doesn't want to.
Anyway the entire point of confining the Grail to the temple is that you're not supposed to have it at all. You're supposed to find it and then leave it. Likewise, you're not supposed to be immortal, and though the grail can keep you alive forever this isn't a good thing.
The ending is about accepting this and letting go. And embracing your family, of course.
After all, Indiana surived when Elsa died because he was able to give up the Grail and she wasn't.
Stepping over the seal without the grail symbolizes letting it go, and thus it stopped fettering the knight's brother. Taking the Grail across the seal symbolized trying to possess it, which is a big no-no.