@
thedeath
From the spider demon i think it is because he cut his own head with his power that he didn't die
Demons still can use their body even if their head got cut off doing so and immediately attaching it back canceling the "death"
And for daki's case it's because she doesn't want to fight nezuko because she found nezuko's regeneration annoying hence waiting for sunlight to kill her later (that's my pov)
and yes crushing their head won't guarantee their death but sunlight does
+ urodotaki ? (Tanjirou 1st master) said that "you can't wound the demon with normal blade" even though they can like muichirou's past he did wound the demon until it was immobilized and by the gods that demon's fate is sealed by sunlight same with the 1st demon tanjirou fight before encountering his master
Such wound will heal in no time unless they are completely separated the body and the head and ofc they need nichirin aka the only working blade to do it