Kamusari and Yoru Were Inherited
Mihawk has apparently had Yoru since Roger died and that would be 24 years ago. He is 43. That means he had Yoru since he was 19. To put this into perspective, Shanks is 39. That would mean that when Roger died he was 15. This is also the age that he had supposedly learned Kamusari.
I suggest that Shanks inherited Kamusari from Roger and Mihawk inherited Yoru from another person. Moreover, Kamusari can be inherited by people talented in Conqueror's Haki and Yoru can be inherited by people talented in Armament Haki. Though, it is rare.
It may be rare for someone to inherit them, but all that would be necessary is for such people to meet Roger and the other person. It would not be necessary for people like Shanks and Mihawk to develop Kamusari or to forge Yoru, but instead talent and luck would assist them.
It seems unlikely that Shanks could develop Kamusari and that Mihawk could forge Yoru. Roger was 53 when he died and Ryuma was 47, yet it is unclear when Roger developed Kamusari and when Ryuma forged Shusui. Is it reasonable for them to do it when at early ages?
Shanks and Mihawk are prodigies. This may be why they had special features early. Although it appears that their developments were unassisted, the special features they possess may actually be the result of fate guiding them toward where they would be most useful.
Note: Kamusari means Divine Departure and Yoru means Night.