for 1st question: you can create javelin as projectile, create bp for character than play montage with delay(for a bit of randomess) and on montage notify spawn projectile(javelin).
for 2nd: animation sharing plugin got what you need( Animation Sharing Plugin | Unreal Engine Documentation ), you can set random play rates and start offset pretty easily through anim bp. I didn’t try this plugin yet, but it will work i guess )