"Scorpion Whip: This whip has a series of razor-sharp blades and fangs inset along its tip. It deals lethal damage, even to creatures with armor bonuses. If you are proficient with whips, you can use a scorpion whip as a whip."
"Items without metal parts cannot be made from adamantine." Blades are metal, so adamantine is okay.
"Items without metal parts cannot be made from cold iron." Same with cold iron.
"Items not primarily of metal are not meaningfully affected by being partially made of mithral." Mithral doesn't work, since it isn't primarily metal.
"The alchemical silvering process can't be applied to nonmetal items" A little questionable, but since you are presumably silvering the blades, it should work fine too.
I'd say you should be able to make it out of mythral, and it would work as silvered blades, like mythral does.
So you should be able t get the mythral whip, it passes DR/silver, but it does not weigh less. I'd say you can make a spear tip with mythral as well, but it does not weigh less cause the haft is wooden.