Does Kha'Zix's Q proc on hit effects? If so, does it proc Feral Flare as well? Does Farel Flare proc on on hit effects in the first place (Ezreal's Q etc)?
Kha'Zix Q is a normal single target spell which deals physical damage and it does not apply on-hit effects. It does however benefit from Spellvamp and It will also apply Rylais slow and the Muramana passive (bonus damage to toggle).
Feral flare will only proc with Autoattacks or spells that apply on-hit effects (Ezreal Q, GP Q, Yasuo Q etc) and not with other spells.
Also a spellshield will block all spells, including those applying on-hit effects.