As to why sacrifice Hawke? Staying behind was exactly what my canon Hawke would do. (Romanced Anders, had to kill him, sided with Templars, etc) He takes responsibility very seriously. It really didn't matter who the Warden was, there was no way he was letting anyone hedge in on 'his job.' Especially not when the Warden still has a shot at cleaning up that mess.
I did just sacrifice a Diplomatic Mage Hawke who sided with the Mages and romanced Isabela and he gave the line about the spiders just like my other Hawkes have. I was wondering if that line could change. It's nice to know that it can, but I'm curious as to what triggers it.
Edit: One Hawke was Diplomatic and one was 'Forceful,' I think the term is? Both gave the line about the spiders. Both were Viscounts, but neither had a current romantic partner. Diplomatic Viscount Hawke was my canon with Dead Anders and the other just couldn't be bothered.