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."
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.
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 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.
Er, Indy had no idea he had any children.
I think the above troper means it in the sense that Indy is a professor and has a responsibility towards his students, who are like his children.
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.
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.
The Leap of Faith trial. There's no way a narrow path could be successfully painted to perfectly blend in with the opposite side of the ravine it spans from both sides at once. If it were just from Indiana's side - the only one that'd really matter for the trial - this wouldn't be an issue, but the camera cuts to the opposite side of the ravine and the path is just as invisible from that side as it is from Indy's.
Then of course that doesn't take into account numerous other factors, such as wind, wear and tear, fading of the paint, wind, and most crucially, the time of day. Unless the cavern was lit artificially, then there's no way that the bridge would blend in with the cliff and it would be completely obvious. Now of course, if the bridge appeared ready to fall apart as you stepped on it, that would still mean a required leap of faith.
The Breath of God trial. Does a penitent man also have to leap and roll out of the way of a second blade coming up from the floor after he kneels? Because that seems like a pretty unfair Kaizo Trap to me.
The first wheel cuts the head off anyone who doesn't kneel—the second is to cut the head off of someone who kneels too far. Kneeling and bowing your head to pray is common in Christianity, but Catholicism didn't require bowing all the way to the ground, which is more common with Muslim prayers. Being that the temple was built by Catholic crusaders, the second wheel is there to catch Muslims who figured out they have to bow down to avoid the first blade.
Holy crap, you actually found a sensible explanation. Thanks!
More likely, the devout know where the switch is on the near side to turn it off.
Which brings in additional fridge logic - how old could that rope he hooked around the gear to turn off the blades be?
Maybe the Knights of the Cruciform Cross swing by to oil the gears, replace the ropes, repaint the bridge, and mop the floors to make sure the letters don't fill up with dust?
The Knights know exactly where the Grail is - they say so themselves. They probably know all about the traps and maintain them - and it's probable they still don't visit the Grail Knight because they're devout enough to have the willpower not to.