The film starting in England makes sense, as time zones would allow Santa to already be on his rounds while it is afternoon when Trudy and her mom pick up her dad.
Santa's powers are all based on belief. When even he is doubting himself, that's when he has problems getting up a chimney or pulling the right gift out of his sack. When Trudy, and later Santa himself, begin to express belief in him, Santa finds himself better able to hold his own in a fight. Where in the beginning one mook was able to nearly kill him, when Santa gains a new hammer, he can now take on a squad of highly trained mercenaries with ease. This is presumably why Scrooge (who believes in Santa) gets defeated by Santa while Thorp (who has no idea who he is) manages to kill him.
Some of the things on the thieves' Naughty List, such as Scrooge killing his own best friend... or the fact that the Extraction team had war crimes and prison camps on their list of bad things.
You can get on the Naughty List for things you never meant to do, if Jimmy/Scrooge is any indication. If you hurt someone by accident or make the wrong decision there's a good chance that the incident will leave a permanent mark on Santa's list. While worse people have ultimately become good (Santa, for example), that's still a terrifying thing to think about for a child.
On the flip side, considering how he interacts with the Lightstone family later, it seems likely that Santa takes intent and the aftermath of those actions into account. Most likely, someone like Scrooge could never get off the Naughty List because he clearly adopted a Then Let Me Be Evil attitude, but someone who ultimately tries to make up for their actions will have their better deeds to atone for their mistake.
