How did the monks know that Isabelle's fiancee stole the beans?
Erm, they saw him?
Jack's locket was air-tight? The bean was in it when he went underwater.
It's not very far-fetched for a locket to be air-tight. Many are, and can keep even pictures dry when people swim wearing them.
The film implies that Jack's world is our world, so how do the Astronauts and airplane pilots not notice the land of giants?
That's an easy one: it's the same reason people don't see it in the sky on a cloudless day. The beans were made with black magic so it seems likely that they are effectively acting as a bridge between two different worlds.
Another theory: maybe they did find the giants again, and, with much more advanced technology, obliterated them and kept it a secret from general public.
Very little space-travel going on in the UK. Maybe we're not as (un)lucky as Jack to have our house directly underneath the Giant's Kingdom.
Why, after they were freed from Roderick's influence, did the giants decide to go to war with the humans anyway? They seemed to have no interest in doing so before then, nor did they act particularly pleased when first ordered to do so.
They thought they could win this time, maybe, seeing as they only lost the previous human-giant war because a human took control of the crown that took control of the giants. With a giant in control of the crown that controls the giants...
The giants were always interested in invading the human lands again because they're greedy as hell and humans are delicious. But they were prevented from doing so by the lack of a beanstalk (remember, it was the monks who grew them, not the giants). Plus, even if they could grow a beanstalk, there was Erik's crown to worry about.
Why in the world was Isabelle engaged to Roderick in the first place? Not only does she make it clear she's not happy with the idea, but Roderick doesn't seem to have any notable reason why the king would think it'd be a good idea to force his daughter to marry him. Yeah, Isabelle had to marry nobility, but surely there were other options besides him?
It was mentioned that he held several government positions, and he might have been a competent (albeit evil) administrator. Maybe the king wanted a son-in-law with proven kingdom-ruling credentials?
Why the fuck does Jack wear the crown instead of Isabelle? They find it together, and she's the one whose ancestor it belonged to. I can't think of any reason for this other than sexism.
He's the one who defeated the giants' leader.
Probably Isabelle wanted Jack to get credit for defeating the giants, because she figures her father will accept a hero for a son-in-law more readily than some jumped-up peasant.
Exactly, what race is Fumm supposed to be a part of? Some people have estimated either black or Aboriginal (personally, I’m betting on the latter). Although, he might he Biracial, since his skin color is nearly identical to the other Giants (who have mostly Caucasian features), and if you look closely, his eyes are a bluish-green color. Still, that leaves a string of more questions, like: “If he’s half white, then what’s the other half?” and “Are there (or were there) other Giants of color as well?”
Since there are no female Giants in Gantua, how do the Giants know to use feminine pronouns when referring to Isabelle? How do they even know what feminine pronouns are?