I think William could be Isaiah. If it's the Rake's imagination as he's dying, and at the beginning of the song it's William killing him. If his daughters came to haunt him he could be scared and expecting enough to imagine Isaiah. Then he dies and is stuck with all three rather than two, and it makes all the rest more interesting, with the queen having resurrected Isaiah from ash ;)
I think William could be Isaiah. If it's the Rake's imagination as he's dying, and at the beginning of the song it's William killing him. If his daughters came to haunt him he could be scared and expecting enough to imagine Isaiah. Then he dies and is stuck with all three rather than two, and it makes all the rest more interesting, with the queen having resurrected Isaiah from ash ;)