-Atk WT!Olwen is worth?

Yes, Bladetomes can make up for -atk more than any other weapon imo.

I agree, that yes she's still worth building. In my experience, Bladetomes often have plenty of Atk that even a superbane won't affect most of her matchups, especially since she should have access to Cavalry buffs.

AND there's always a chance that you'll pull her again, since there aren't many units in the green pool and there's always the slight possibility that she gets demoted to 4* after this banner.