Basically, Kyp Durron is obviously on a far, far higher level to Maul in terms of Force ability. Maul has nothing that compares to Kyp rotating the gravitational singularity of the dovin basal, using the Force to oneshot the leviathan, doesn't have the benefit of statements about his dwarfing potential on Kyp's scale. Kyp's monstrous Force ability just overshadows everything Maul has done in this department.
So, going by your own idea:
Moreover, proficiency with a lightsaber to some extent runs in tandem with Force power if the Force user is a devoted duelist, which both Caedus and Revan are, so if Revan is more powerful, it makes little sense if Caedus was above him as a duelist. Especially considering Revan learned all he could from the Jedi Order, which includes all lightsaber forms.
Kyp is a very martial Jedi, he's even classified as a Jedi Guardian. He's a war hero, he was the first on the frontlines of the war of all the Jedi in the NJO during the Yuuzhan Vong War and is one of its most accomplished combatants. Thing especially worth noting is that the Jedi had to rely on lightsaber skill throughout the entire war, since their normal abilities didn't work on Vongs. Times forced the Jedi to be devoted to dueling, because it was the only thing that worked.
Kyp may just lack the talent, the knack for fighting, and be a pretty shitty duelist for someone of his Force power, but the gap is just too extreme and Kyp simply has to be Maul-tier combatant based on your own idea.
Now, a Slayer so weak he died completely on his own, dropping dead minutes after the fight, still managed to defeat Kyp.
Even if we go with the idea that:
- Kyp had a really, really bad day, really bad position in the fight,
- Slayer had insane luck,
- he was the best of all the Slayers,
that still means the random, healthy Slayer is above Kyp, and above Maul as well.