The movie opens in "Aarhus, Eastern Denmark". Aarhus is actually the capital of the westernmost part of Denmark, Jutland (though it's on the east coast of Jutland, which is probably what confused the script writers).
After Arthur hooks on to the upside-down turned sleigh, it takes him over a minute to get it under control and make a U-turn, but less than 15 seconds to make it come to a full stop. Thus it couldn't possibly have returned to where the boat was, and crash on top of it.
Scandinavia celebrates Christmas on the eve of the 24th. So even though Denmark is apparently Santa's first stop on Christmas night, the children would have long since unwrapped their presents when he got there.
Steve's tablet states that he has completed Hungary. However, according to Hungarian tradition, Santa visits Hungarian children in the night of 5 December, and Little Jesus brings gifts to them already in the evening of Christmas Eve.
After Arthur re-summons the sleigh with the help of Grandsanta, Grandsanta takes his sleigh, Evie with Arthur & Bryony, up into space to get a clear map of the world in order to find Trelew, they are only wearing breathing masks. If Arthur, Grandsanta & Bryony were beyond Earth's atmosphere without any protective astronaut clothing they would be completely exposed to lethal doses of radiation from the sun and their body fluids would fizz and bubble if they was wearing Santa & Winter Jacket clothes, even if they were just up there for a few minutes.
Arthur falls out of the sleigh as it flies over Cuba. Several seconds later, the gift-wrapped bike falls out, landing directly in front of him. Given the speed at which the sleigh was flying, the bike should have landed much further away.
When Arthur jumps out of the sleigh into Trelew, England to get to Gwen's house in time, he uses Santa's toy sack as a parachute. He has the bike grasped to his body but when the toy bag turns upside down as he's floating to the ground, a toy falls out meaning that they missed not just 1 present but 2, and that they didn't just miss Gwen.
After the plane destroys EVIE, the S-1 soon appears with Malcolm, Margaret and Steve. However, it is impossible that the plane didn't notice the S-1 appearing close to it, as the S-1 is shown to be visible from above. If the UNFITA can detect electronic devices, such as the slippers, they also should have noticed the S-1, which has a technology "more developed than Pentagon's".
Grandsanta says that he is 136 years old. But if he were 136 years old, he would not have been able to be Santa for 70 Christmases and then see his son, the current Santa, complete 70 Christmases as Santa. However, as mentioned in the beginning of the story, the portrait for Santa Claus the Nineteenth(Grandsanta) completed only 39 missions from 1902-1941, making him 27 years old on his first mission.
Even though Gwen only asked for one gift which was the bike and seeing how Gwen is an only child, there appears to be tonnes of Christmas presents under the tree, and surely her Parents couldn't have asked for that much.