-Atk +Res Olwen?

Yes. Most Bladetome units don't have to worry about attack banes.

I'm pretty sure yes, as long as you give her hone and fortify cavalry she gains +30 atk (+6 for the direct buff to atk, +24 because Thunderhead is basically a blade tome) so -3 atk isn't going to feel like much.

By comparison, the 4* version of a blade tome is sufficient to do so much damage it's not really necessary to promote a Nino or Tharja or Odin or Tailtiu to 5* for the extra 4 atk.