That's because Jahad family considers it as a taboo. We will definitely see more of it later.
No, I don't but they will definitely have a hax similar to spells. We know Adori has Arie swordsmanship. Anne should also have inherited sth from blossom. Tho, I am more curious about Baek.
This clearly tells us Armour is the reason behind the immortality of Dumas and Karaka.
Well if we just look at slayers, everyone have sth to do with spells. First Mirchea, I don't think I need to talk about him. Then White, he became slayer because of his ability related to devouring souls which is thanks to spells.
Then Karaka, while he became a slayer (we don't know if it was without armour as JH mentioned he had an armour before he gifted him), even if he didn't use armour to become slayer, the reason why he can even come to the battlefield with strong HRs compared to him is because of the armour.
Then there's Yama, while doom was chosen a slayer initially, Yama got this seat. Even in cage, KH wanted to make doom slayer because of the spell installed in his heart. While it doesn't give any power to doom, spell is another reason why he almost became one.
Then as a bonus, here's reaction of Dumas to Khun as a sorcerer.