I think Pet Sematary is one of King's more horrifying works, but I don't know how effectively you could make the horror of Gage being a killer work well in a movie. My understanding of the first adaptation is that it's lots of awkward special effects, and what they'd need to do is get the kid as he is in this film, a toddler who isn't acting, and yet still have him do a lot of things it would be very difficult to do, from the stabbing to walking like an adult in a toddler's body to all the other things that you can imagine, but recreating would be near-impossible to do effectively.
To
@Bush did USS Maine 's questions, the first truck scare is foreshadowing that the road is dangerous because of the carelessness of truck drivers. The point of the toddler running towards the road when the girl is right there is because in the book, it's the boy who dies, not the girl, and so anyone who knows the story is expecting him to get crushed, not her. John Lithgow's motivation is that he was well-meaning, but even though he knew the Pet Sematary was a terrible spot, once you know about it you feel a sort of compulsion to spread that knowledge to someone else, rationalising it and talking yourself into it because the place itself wants people to use it, to bring them back.
The black ghost dude and the Wendigo? Don't know. Don't remember the book well enough. Maybe he felt the need to protect the doctor but not the others? There was a thing about how if you bring someone back, they also come back
yours somehow, which might explain why Ellie seemed fine with her father at first but immediately wanted to kill her mother. As for undead being stronger, they usually are, as well as not worrying so much about things like pain. Add that to the difficulty of fighting something that looks like a loved one, and them being armed while you're not, and trying to protect the boy while doing so, and I get why the fight ends the way it does.
Overall, because I didn't remember the fine details, just the overall plot, I didn't mind the changes because it kept me on my toes as to what could happen, as well as making the toddler a constant source of concern; I don't remember how the older sister fares in the book, I'm presuming not well, but we expect even a child of six or seven to able to fight for self-preservation - run, hide, follow basic instructions, call for help. But a toddler can do enough to get themselves in trouble, unlike a baby, but not enough to actually protect themselves.