If Camus has Granni's shield equipped (which neutralizes "effective against cavalry" bonus damage,) will cavalry specific buffs such as Hone Cavalry, Ward Cavalry, etc. still work on him?
Yes. So Granni's Shield just neutralizes bonus damage from anti calvary attacks, so that it calculates damage normally. Hone Cavalry, Ward Cavalry, etc. aren't "effective against" cavalry, they are buffs. Buffs are not normally detrimental, not to mention damage reduction and applying buffs are not the same thing.