There are mistakes that can be forgiven and there those that can't. If someone is going to make such a grave mistake, it is uforgivable, and is an idiot in my eyes.
Poeple asked here why some pepole don't like hawke. There's your answer. I don't like to play idiot protagonists, and Hawke is one of them.
So you suggest that instead of letting Leandra live and enjoy her life, like the adult person she was, Hawke should have imprisoned her in the manor, because she might be snatched up by a serial killer? Yes from a metagaming perspective (especially if you are used to Bioware trying to rip out your soul via drama), Hawke obviously should have suspected foulplay, as soon as her/his mother decided to move on, but from the point of roleplay, no it wasn't.
Kirkwall is a city full of people, I am pretty sure it might have also been full of men, women who send their lovers lillies, without planning to kill them. Otherwise the fault lays with the guards, cause they should have just arrested anyone owning lillies, as they were up to no good.
Regarding a possible comeback of the warden. While I might wish for it, I don't see it doable in any kind of enjoyable setting, since -like stated from others before me - the Warden is even more our own hero, then Hawke and even if they found a way, to capture atleast certain aspects of our wardens personality, it propably would still result in an incredible cry of outrage, by those who believe to have done the right thing, by refusing the dark ritual and now don't have any warden to go on. So I will try to be content to just hear about my warden 