Here's what I understand :
If I understand correctly, characters on monsters (or chariots) are affected separately only for the purpose of effects that inflict wounds.
For the rest of game purpose, they are a single model (And, a character).
That would mean «removed as a casualty» effect removes both rider and mount (with no saves of any kind), and augment and hexes affect both.
Also, Okham's mindrazor allows mount to strike with the leadership values of their rider (or the general's inspiring presence as per FAQ). After all, the BRB specifically says that the leadership values of the mounts are never used and that they always (that's always) use the leadership value of their rider.
Also, because it's a game, it's perfectly acceptable (and even desirable) if, sometimes, rules interact in somewhat non-sensical and cheesy ways. Games such as these, after all, are good practice for who wants to be a lawyer and a game without cheese is no fun.
I fled to chase dreams that I can't yet describe.
I wake up, shivers, I hope my dream wasn't you.
But son, your dad, it ain't me, it can't be.