Alexander uses Manannan's same spells: but he uses them in positive ways to solve problems. We can assume Manannan most certainly didn't.
How Alexander defeats the dragon is very much along the King's Quest philosophy: you don't go in and confront the problem, sword in hand... you find a clever way to deal with it in a non-confrontational way.
Keep in mind that the very first King's Quest featured a dragon guarding the Magic Mirror: you could deal with him directly with the sword (having the game chide you for picking such a violent option, and getting far less points) or use the water bucket to drive him away (netting more points and avoiding outright battle).
The AGD version features a splash screen of Alexander's victory over Manannan, holding the cat by the scruff as it dangles helplessly: Manannan is now the one who is completely at the mercy of a captor he cannot harm or escape from.
Gwydion has spent the last seventeen years in the same house, only allowed to go out into the yard to feed the chickens. And even the house has several areas off-limits to him. He has spent his entire life in roughly 500 square feet of space.
Looping right back around to Fridge Brilliance: how does Gwydion/Alexander so easily rebel against Manannan if he's never been let out of the house? He's snuck out before. Manannan clearly needs to travel a lot, and Gwydion has been taking advantage of that, but he's never needed to worry about his safety in Manannan's house (well, his life at least, as it's clear that his safety is massively undervalued by the wizard) until just before he turns 18.
Manannan and his original cat are probably in for a long, lingering death by starvation and dehydration, as the door is always closed in the house and there is no apparent way for them to escape.
One of the rats on the pirate's ship says he misses his many cousins who never came back from the lab. People have been doing testing on animals for centuries, but it's still unnerving and sad to hear the cute little rats Gwydion can now understand say that.
Those pirates will not awaken from that magical slumber. They will be dead in a few days and that ship will become a ghost ship.
In the AGD remake, the previous Gwydion knows exactly what will happen to him the morning of his 18th birthday. Unable to escape, he is simply waiting for the morning on his last night, knowing what's in store when Manannan next summons him in a few hours. He is right.
The enchanted kingdom of Daventry has become a fiery hellscape.
King Graham, the legendary adventurer who has overcome so many impossible odds, cannot save his kingdom OR his children. Without Alexander's heroics, chances are good that Graham would have soon killed himself.
Even with the happy ending, Alexander's entire childhood has been lost. He's a man now and all of his memories of his early youth will be of drudgery, toil and fear. Many potential happy years of being in a privileged, wholesome and loving family are lost forever.
