In both film versions, keeping the monkey's cymbals from clashing is enough to stop its curse from taking a life. There's no guarantee the little bastard wouldn't get around it somehow, but why did nobody think to at least try blocking its cymbals with a ping-pong ball and some duct tape? The Stephen King short story the movie plagiarizes did address this, unlike the film; the protagonist specifically mentions that he tried muffling the cymbals and it didn't work.
Merlin's wife tells Mr. Cooper that they don't give demonstrations... only a few minutes after giving a demonstration to a kid. They can't even deny this since Mr. Cooper saw it, asking if it was the extent of Merlin's powers.
Maybe if Jonathan wasn't such a colossal ass about everything, they'd have been happier to oblige.
If anything in this movie can be called "brilliant" the closest thing would be what at first seems like one of the biggest plot contrivances of the movie - how did Michael's dad manage to survive falling into the giant fissure in the Earth, as well as having a tree fall on him? It's because the evil monkey can only kill if it can clash its cymbals together. It could use all of the evil magic it wants, call down earthquakes and storms, but because its arms were tied together it wasn't actually capable of causing death no matter how hard it tried.
I think the actual explanation is that the protective amulet the psychic gave him stopped the monkey from killing him - the next scene has him take out the amulet and put it in a drawer, with the camera focusing on it and everything.
When I first watched the episode of Mystery Science Theater 3000, I wondered why the spell turned Jonathan Cooper into a baby. Then I realized something: earlier in the episode, you could see that 5 drops of human blood were needed on the page where the youth spell was located. In his haste to get younger, he took way more than 5 drops from his wife, thus causing the spell to not work right. In the end, he suffered babyhood due to his inability to follow the directions.
Just how is Madeline going to explain her husband's sudden disappearance and her new baby? Especially after she spent the whole of her time onscreen reminding everyone within earshot about her infertility? No one would believe the real explanation (magic) so it's more likely everyone's going to assume that she murdered her husband and stole a kid.
On the other hand, Madeline presumably found the handful of diamonds Johnathan created while he was messing around with the book and could use the money she got selling them to disappear... though that raises other questions, like how could she possibly pass those diamonds into the economy without raising a LOT of eyebrows?
Johnathan Cooper is probably going to kill his wife/mother. Even though he was age-regressed back to infancy there's nothing to suggest that he doesn't still have all of his memories and knowledge. So we have a guy who was already a Jerkass, turned into a kid and being raised by a woman he already didn't like, and he knows magic. (Even if he no longer has the book, he should still remember a few spells, especially since he was speaking them into his tape recorder the whole time.) Mike Nelson was right: clearly, Merlin has brought good into the world.
An infant's brain simply isn't large or complex enough to store an adult's knowledge and personality. So it's more like Cooper is dead.