Karma's a bitch

Fodder her A skill, is a free Death Blow.

Keep the +Res one and fodder the other for the extremely rare Drive Attack. I had one at lv40 but got a better one, +Atk -Def, and leveled that one up with the old one going for Drive Attack at some point.

Merge them. Because Delthea is better at Nuking than Reinhardt, plus you have other easier to obtain characters like Hawkeye and Klein if you really need Death Blow fodder. I would recommend +ing the +Res one as defense on her is worthless and you could use something like Iceberg/Glacies or Moonbow to make up for the -Atk as well as an Atk+3 Sacred Seal

...that's tough. All 3 of the options given are viable. I have an unmerged Delthea and she mostly does fine, there have only been a few matchups where she falls short, and I don't think two more stat points would've changed that (but you will gain one hp and one atk by merging). If you do keep one or merge, make it the +res: she has one of the highest resistance stats in my barracks so she's quite useful for that reason. Her defense can't be redeemed.

If it was me: if I had a unit I was maximizing for its arena score, I'd add drive attack. Actually, I'd sack an Ogma to that unit first for the two spur attacks, then I'd inherit drive atk 1-2 and death blow 1.