Well, obviously Daphnes can't be dead because of the Triforce...Which sucks, because I think the idea of him being a ghost makes the most sense...
Anyway, no, I don't think he's the king from
OoT. Ganondorf was implied to have killed that king.
And Daphnes knew both Fado and Laruto, as he conducted their songs to the Master Sword with the Wind Waker. Since Fado and Laruto weren't in
OoT, then a period of time has passed since then, and that means Daphnes couldn't be the king in
OoT. He was probably the king during the flood period.