Continuity mistake: After mowing down all the Immortals during the Sanctuary scene, one Watcher tells another to take their heads because they might be Immortals. You see the Watcher to the left side of the screen and a robed figure to his left about four feet away drawing his sword. Suddenly, that same robed figure is now to the Watcher right side and about two feet away.
Continuity mistake: During the final duel, Kell traps Duncan's sword against a guardrail and then kicks it into the air. Duncan jumps off and grabs the sword with his left hand. After he hits the ground, the sword is suddenly in his right hand.
Continuity mistake: In the part of the movie where Duncan MacLeod is on his way to Connor's old store, he is shown first riding in a 1998 or newer Ford Crown Victoria taxi. When he is shown getting out, he is exiting a 1992 to 1995 Ford Taurus taxi.
Continuity mistake: While Duncan is riding in the taxi, the numbers on the side change from shot to shot.
Continuity mistake: When Kell is stabbed by Connor at the beginning of the movie, the sword is sticking out from his back about one and a half to two feet. Yet when he lands the sword doesn't seem to matter since he is laying flush on his back.
Continuity mistake: After Joe shoots and kills Hale, it cuts to a wideshot of Duncan and Kell on a catwalk squaring up against each other. Duncan makes a very specific move, where he glides his sword from his right side to his left. The shot then cuts to a medium shot... and Duncan's sword is suddenly on his right side again, and he makes the exact same move a second time.
Continuity mistake: During the flashback to 1712, Connor and Duncan fight it out with highwaymen who are collecting fake tolls from travelers. Throughout this scene, Duncan McLeod's five-o-clock shadow starts out dark, becoming lighter, then darker, and then almost clean-shaven from shot-to-shot as the fight progresses. This continuity problem persists, to a lesser extent, throughout the film.
Continuity mistake: In the series season 3 episode 8: Obsession, Anne asks Duncan if he's ever been married and he says no. In this film you find out Duncan was married to Kate/Faith.
Answer: The scenes you mention were only meant to be part of the trailer. This was to show that Jacob Kell was the "most powerful" immortal yet. The 'cut in half' scene is playing on the idea that Kell is unbeatable. I suggest rewatching the trailer with these ideas in mind. It may make more sense. These scenes were what Dimension Films wasted a hefty portion of the promotional and marketing budgets on. There was NEVER any script written that included them, nor any version of the film that ever included them.