It's been a while, but I'm gonna pretend my brain is functioning again...
#5: Roe + hen + y, -... well, either an R, E, or N for a direction. It comes quite close to making ROHERYN, losing the E, but then "a wherefore" needs to be RY / YR, which I can't quite make fit. Also, (4) wouldn't indicate either the spelling or the book/chapter he's in (though could be 'what was the last Age he lived in), and, er... I don't know that Roheryn is even a male horse.
hS