Inferno Stones only use as a weapon is that the orb itself explodes, it can't be used to power up a weapon. Carving one into a weapon would be the equivalent of inserting a grenade into a gun.

@Rigel31415 Inferno Stones can enhance flames in a forge without explode. It should theoretically be possible for it enhance flames of magic weapon.

Inferno Stone itself isn't being used as weapon, instead used to power up another weapon.

It can't. What Welf used as fuel was the Amphisbaena's drop item, not the Kaenseki.