When the Grail Knight says, "He chose... poorly", he's not referring to Donovan's selection of the Grail - he never picked the Grail, Elsa did. The Knight is stating Donovan's choice of allies was poor.
The line works either way. Donovan still believed the False Grail was the real thing. "This truly is the cup of the King of the Kings" and all.
However, Donovan still had no idea what the Grail would look like. He would have said that had Elsa picked the actual Grail.
He didn't choose a grail. He chose to let Elsa decide, and then he chose to listen to her. And he never chose to ask Indiana Jones, who up until that point had found a way past all the other traps and tricks that had killed everyone else before him. Above all else, he chose not to decide for himself - the worst choice of all.
He might have noticed that Indiana was obviously struggling to keep his mouth shut. Hero that he is, Indy was probably sorely tempted to tell him, "That's not the Grail."note Indy being an archeologist first and foremost.
Had he not be admiring the cup, he might have seen the very sour glare she gives Indy as if to tell him, "I know it's the wrong Grail, but I want to Kick the Son of a Bitch."
He also chose poorly in his interpretation of Jesus. Yes, He was the King of Kings, but during His time on Earth, He was also a humble carpenter.
Elsa also chose poorly, both literally in choosing the wrong grail, and in choosing to knowingly let someone drink from it to their doom, but also in choosing to take the grail with her and choosing to keep seeking it when she should have abandoned it and lived.
Elsa was visibly shaken earlier when Henry Sr. got shot, even if she wanted the grail for herself she didn't want Donovan to get off that easy.
He also chose to drink from the cup. Had he simply taken it home and put it on his mantlepiece as a souvenir, nothing bad would have happened at all most likely. (Though we saw what happens when you try to cross the Seal with the Grail.)
Since it wasn't the right grail, he might not have been punished had he tried to take it with him. He'd have gone home with a pretty souvenir, but it wouldn't have been what he was actually after.
Henry Sr. talks Indy out of trying to get the Grail by calling him Indiana. Not only is he recognizing him as an equal, by calling him his preferred name, but Henry Sr. had spent his whole life looking for the Grail, and he's decided it's time to go on without it. And afterwards, when Indy asks him what he found at the Grail temple, he says "Me? Illumination.", which is a Call-Back to the beginning, as when Henry ignored him while working on the Grail diary, he said "May he who illuminated this, illuminate me...".
When Donovan recruits Indy to look for the Holy Grail, he says "Find the man, and you will find the Grail.". He not only speaks of the Grail itself, but Indy's relationship with his father, with whom he's been estranged for years. It's this search for the Grail that brings them together.
Steven Spielberg said it himself in behind-the-scenes interviews, "the search for the father is the search for the Grail".
Indy hates his father because he was never there for him growing up, because he was working on his grail diary. But as an adult, despite his Papa Wolf reputation from the previous film, Indy's never there for his children (his students) either! As a professor, he has dozens of students wanting to speak with him about his stacks of ungraded exams and papers and numerous phone messages, because he was focused on his own pursuits, such as the Cross of Coronado. And even in his adventuring life, he apparently sent his Kid Sidekick Short Round to boarding school and seldom if ever saw him again.
The results to choosing poorly with the Holy Grail is a no brainer for Catholics & Orthodox. This is why they are so picky about Communion. They aren't being unwelcoming. They are trying to protect those who don't know the dangers. To wit:
So then, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of sinning against the body and blood of the Lord. Everyone ought to examine themselves before they eat of the bread and drink from the cup. For those who eat and drink without discerning the body of Christ eat and drink judgment on themselves. That is why many among you are weak and sick, and a number of you have fallen asleep [in death]. (1 Cor 11:23-34 NIV)
Indy is implicitly or explicitly portrayed as a knight multiple times throughout the film, such as when he uses an improvised lance against a Nazi motorcycle-trooper, or when the grail-knight refers to him as such during the film's climatic scene. The scene with the tank takes on increased significance in light of this, as it portrays Indy as a lone hero on horseback fighting against a massive, unstoppable, firebreathing (in a manner of speaking) monster.
Also, tanks replaced armored knights on horseback, so this is Indy fighting against another, stronger knight, relying on his wits to outmatch his opponents superior strength and arms because his cause was just.
Why Henry was able to hold up Indy from falling despite early on not being able to take on a Nazi around the same age in a fight? Because after drinking the grail he didn't just get healed, the grail made him younger too and more stronger.
Well, it also probably takes skill and strength to fight a soldier, and it only takes some strength and leverage to hold up a younger man's weight. Plus, Jones Senior seems to be in pretty good physical condition for his age.
Henry Sr was braced, and even with that added advantage, he couldn't bring Indy's slack weight up — he needed Indy to co-operate.
At the end of the movie, Henry's hat comes off after he's been shot, and he ends up leaving it behind in the temple. He has to improvise a new hat to protect his head on their ride into the sunset—this changing of hats represents Henry becoming a new person as a result of the "illumination" he experienced in the Grail temple.
Watch Henry's hat throughout the movie. Whenever he's being his gruff normal self, he's wearing his hat. EVERY single time he has an emotionally vulnerable moment, the hat comes off.
The whole subplot between Indy and his father becomes this to those familiar with Grail lore as a big part of the Arthur legend was the reconciliation of Sir Galahad with his own estranged father, Sir Lancelot.
Henry being wounded by Donovan allows him to fulfill the role of another figure in Grail lore, that of the Fisher King.
"This belongs in a museum!" "So do you!" Indy is a professor of archaeology, and really probably does belong in a museum. This line lampshades some of the more outlandish elements of the Adventure Archaeologist genre.
When Donovan crumbles to dust, the dust blows away and all that's left of him is his swastika pin. He claimed that he was just working with the Nazis and thought of them as allies of convenience, but like Elsa, when it counted he stood up with the enemies of everything the Grail stood for: beneath his excuses, justifications, and delusions, he was just another Nazi.
See also the fact that no one forced Donovan to drink from a strange cup that someone had picked from a whole range of strange cups in a dusty cave in the middle of the desert. He could have just taken it home and kept it on his mantlepiece as a nice souvenir. It was his choice to drink.
Why does Donovan continue to cling onto her screaming even after he's almost entirely rotted away? Because on some level at least, whatever's left of him realises what she did and is probably giving Taking You with Me a try...
Given the location (the final resting place of the Holy Grail) and the reason for the pit's opening (greed/lust for power) it's entirely plausible that the pit Elsa fell into leads directly to Hell itself.
While the famous "No ticket!" scene is considered a funny moment, it can have some horrifying elements. Observe how quickly the other passengers reach for their tickets, and consider the scene from their eyes: You live in a country that has been forcibly seized by fascists who begin to do terrible things to people that you may have known for decades. You book a luxury trip (maybe an escape...) on a vehicle that should be safe, when a large man throws another man out to his death. This man turns and explains that the unfortunate evicted had no ticket. Is he a Party member in disguise? Is this another SS trick? No wonder they were so eager to show off their right to be on the blimp, they didn't want this soldier to kill them as well!
Donovan's line about the treasures brought to bribe the king being "donated" by some of the finest families in Germany becomes much much darker when you're an adult with an understanding of history and you realize exactly what he means.