It'd be wiser to focus on your starter champion rather than focus on Executioner or Relickeeper. Any of the 4 starter champs would just be better all around (especially Kael) and could clear Brutal 12-3 faster. 

And no- neither can clear Nightmare campaign. There *may be* a slim chance with Executioner if paired with another crazy geared champion, or Paragon. NM campaign is actually very difficult and I suggest not to focus too hard on getting all 3* right away, you'd be farming campaign on Brutal anyway. 

6star kael. Do nothing with the other 2