Dark Urge and Act 2 point of no return (spoilers)

[Steffan]

My character will probably not kill Isobel but he still hesitates a bit. Is this mission only available until you deal with the Nightsong and the Ketheric relic (my guess is a sort of philactery)?

Thanks for the information. No major soilers please. ;)

[Winter Light]

It's available until you enter Act 3 I believe. At certain points in the story killing her becomes far easier (you can catch her alone) so I'd hold off until the right moment if you want to do it.

[Doomvora]

As long as you don't long rest and don't leave act 2 (after fighting ketheric) you will not fail the quest.