Joshua Speed outlived Abraham Lincoln by about twenty years, and died in 1882. Did the moviemakers forget that he was a real person? Maybe they thought his name looked too fake.
Yes, just like they forgot that vampires didn't cause the Civil War.
Why didn't Lincoln coat the entire shaft of his ax with a coating of silver, that bowl he and Henry used to coat the ax blade edge would be enough to at least make some silver stripes running down the light of the shaft that would prevent vampires from holding it and make the ax a much stronger weapon.
Also, in the film at least, why didn't Abe have a silver coated ball in his hidden gun during his fight with Jack Barts?
Why did the vampires spare the Union officer at Gettysburg?
They needed someone to deliver the news of their crushing defeat.
How come no one saw a problem with a train crewed by only the US President and two of his friends go to Gettysburg despite supposedly carrying ammunition vital to the war effort?
I want to say they were somehow keeping the whole thing under wraps, maybe concocting some kind of cover story for why they were seizing all the forks and knives out of everyone's homes or something, and then once they get to Gettysburg, they just sort of go "hey, here's some more ammo for you guys." It's kind of hard to iron out all the details of how it works out, but I think that's the general idea: Lincoln and his friends are the only ones who even know the plan.
There is also the fact that Lincolns Assassination is the catalyst for the start of the President being monitored closely and having bodyguards, heck, Lincoln would walk the streets of D.C. by hIMSELF, with no guards or support anywhere near him. No one thought a president would be assassinated, heck, most of the Confederacy didn't want him dead, they just wanted him to not be President anymore.
I only saw the movie, and it may have been explained in the books but I have to ask anyway. Why does Henry want Lincoln to be a vampire and hunt vampires forever when vampires aren't allowed to kill each other?
Because personally killing vampires with your own hands is not the only way to combat them. From the movie itself, we already have Henry, who's been busy training hunters and otherwise working against the vampires for however long he's been a vampire, and Lincoln himself, who shelved his ax for decades and fought the vampires with nothing but politics, all the way up until the train scene.
I don't remember this rule existing in the book's universe. It seemed like they just threw it in for that one scene. There's definitely a bit in the book where vampires fight each other.
The author did in fact add that rule in for the movie version, though it may or may not technically still work with the book version of the story (I can say since I haven't read it yet) depending on whether vampires ever outright kill each other in the book. In the movie, they can still very much smack each other around, but some kind of magic force field blocks them from landing lethal blows, e.g. Henry's knife.
Henry outright kills a few vampires in the book. I think the reason this rule was added is quite simple; to answer the question of, "if he can kill vampires on his own, why does he bother training or recruiting humans to do it?"
Well, actually, whether or not Henry could kill vampires himself is moot in the book when it comes to why he trained Lincoln. He took Lincoln on and trained him (after saving his life) not only so he could have help killing vamps but also because Lincoln wanted revenge for his mother.
In the book, Abe had already avenged his mother by killing Bart years before he met Henry. He had taken hunting vampires as a personal crusade all on his own, basically becoming a vampire-hunting batman. Henry saved his life during his second encounter with a vampire and decided to train him partly out of pity, partly out of amusement, and because he suspected Abe would be a useful tool in the proxy war that he knew was brewing. In the movie, Henry specifically denounces the concept of personal vengeance as a motivation to hunt vampires, flatly telling him that if that's his only reason for doing it, then he's wasting his time.
If Adam has been alive for thousands of years, witnessing, at one point, the Egyptians enslaving the Hebrews, why is he white? Wouldn't he be, you know, brownish?
He could be still be 'white.' I don't think he ever really mentions where he is from in the world; and even if he were Egyptian or Hebrew he doesn't automatically *have* to be brownish. Ramses was a redhead after all.
Where the hell does Will get super-vampire fighting skills? Or for that matter, not visibly age aside from grey hair. Was he secretly a vampire?
If he was a vampire he wouldn´t be able to kill vampires. Just a case of Badass Normal.
So seeing that a vampire can shove a knife at another, could a vampire be able to kill another vampire with a gun or other ranged weapon? If not, would the bullet be prevented from impacting or would he be unable to pull the trigger?
He'd be unable to pull the trigger. It seems to work more as a psychological block that stops you from taking actions to intentionally kill another undead rather than a magical forcefield sort of deal - Adam says he's also under the effects, and he has no problem beating ten shades of shit out of Henry just so long as he lives.
In the movie the vampires can walk in daylight thanks to the sunscreen. Then how comes Henry after being bitten by Adam and turned into a vampire doesn't disintegrate since he's not using it (the sunscreen) ?
They're never shown disintegrating. It's possible these kind of vampires are only in pain when exposed to sunlight. Sunscreen and shade make it less bad. Henry was probably affected too much by the emotional pain of helplessly watching his wife be murdered to feel any physical pain.