A few indecisions about healers.

Definitely use the -Hp one, -HP is the easiest stat to throw away unless you're running an important skill that works better the with more HP. So yea the -HP/+Res is the one I think I'd go with. The only reason to take the +Def one is if you legit expect her to be tanking a lot of ranged melee, but I think in general tomes are the bigger issue.

I put the Savage blow build on my Genny and it's amazing. If you plan on using her in Arena, then merge her. Also keep in mind that healer weapons can be upgraded to wrathful via weapon forge which frees up the B slot for something else.

Whenever I get units that I'm not sure what to do with and I think I have a few ideas of good uses, I typically decide to do nothing and hold on to them. I might suggest this with your Candlelight since it is going to be rare and you aren't super enthused about a use yet. One day you may have the *perfect* use for it when something new comes your way.

As far as Mist, I'd use her for C-skill fodder on a much better support unit.

Genny’s Spd is way too low to make use of Close Counter. She will be doubled by a majority of units, and her bulk might even result in being oneshot.
Furthermore, her Res does not synergize with Close Counter. The only magic melee units are dragons, and they’ll always target Genny’s lower Def.

Tbh I would kill mist for Recover+, Miracle and Slow+ for another healer. Maybe even for your Genny? Just learn Recover and Slow from a Serra and Wrys and you win 40k feathers if you ever wanted to build a healer with those. Double spur can come from a 4* Morgan, so it isnt worth it to sack her for that