Other mistake: When Allan is teaching Sawyer to shoot, he says that he 'brought his son along' on a mission. However, the subtitles say 'son-in-law'. (00:44:35)
Other mistake: Before Quatermain shoots the fourth attacker long distance he puts on his glasses, cursing his getting old. However, when he aims he looks over the rim. (00:11:05)
Other mistake: When Sawyer fires at the men on the roof in Venice, he shoots over 30 bullets from two six-shot revolvers.
Other mistake: When Quatermain impales the henchman at the beginning of the film, the rhino is hung high on the wall and the tip is much higher than the spot the man hits, Quatermain would have to have lifted the man higher than he does to achieve this.
Other mistake: In the scene when the League are attacking the factory, we see Captain Nemo demonstrating his skill with his sabre. In the last shot before the scene changes, he cuts down a number of people and then sheathes his sword all in one movement. The camera angle is from the side where his scabbard is hung, and although you can hear the hissing sound of the blade being sheathed you can see the sword completely misses the scabbard and is just stuck out behind him. This is obvious because the sword is silver and the scabbard is black.
Other mistake: In the scene right after the team gets a Morse code message from Skinner, we see the holes made in the Nautilus. The one in the middle is smoking whereas the one in the back is not. If the ship was filling with water to the point that is was immersed due to the holes then whatever explosive components would be wet and extinguished. So by the time the Nautilus resurfaced, it should not have been smoking at all.
Other mistake: When Sawyer tries to take a shot at the Phantom just before he escapes from Dorian Gray's attic, Sawyer fires once, but two impacts are kicked up from the dusty floor. And the impacts come in to rapid succession for a manually operated rifle, even with a crack shot using it.
Answer: I haven't read this book of Wilde, but I think alterations were prompted not by copyright issues but because this conveniences the "league" storyline.