Character mistake: The crew many times refers to the sub as a ship - subs are always called a boat.
Character mistake: In the courtyard of the castle there is an anti-aircraft gun. You could probably not find a worse place for a gun like that even if you tried (except maybe if you put it inside, in a windowless room). In the position it is placed it can only fire at targets directly above the castle. What you want to do is fire at targets before they reach the castle. It would be better if they placed it on the roof, or even near the village below.
Character mistake: In the saloon on the way to Sweetwater, where Harmonica meets Cheyenne and his gang for the first time, Harmonica grabs one of the men's coats and looks at it, saying "I saw three of these dusters a short time ago. Inside the dusters there were three men. Inside the men there were three bullets." Actually, while the movie starts off with the three men at the train station wearing dusters, in the shootout scene, the black cowboy is no longer wearing a duster. Harmonica would have only seen two dusters. (00:41:50)
Character mistake: A space traveler would not be "going to Orion." The major stars in Orion vary in distance from the earth from 243 to 1,360 light years. Orion is only a constellation as visible from Earth; once you get away that far, the stars would not be in the same alignment as they are from Earth. They would have been going to a specific star, probably Bellatrix at 243 light years, as Taylor mentions being at 320 light years.
Suggested correction: I'm not sure what scene you're referring to, but I never heard anyone say they went to Orion. Taylor says they're on a "planet in orbit around a star in the constellation of Orion. That could be Bellatrix." To Taylor, who is from Earth, the star is still in the constellation of Orion.
I agree with your take, but I still think it could be interpreted as Orion's stars all being in close proximity. And I would hope they would know about the 320 - 243 light year issue.