I was asking a lot and thinking hardly about these two nice ladies and finally I created my own opinion.
Scadi is still more Quick looping support than buster crit and my only good quick buddies is Jack and Altera Santa.
Scadi totally doesn't have any protect, not for team, not for herself.
If you have a fat copy, Scady also may become a DPS, but I think her main niche is farming.
But in farming you always can use friends, so, if you don't have a lot of sq, Scadi don't worth summoning, if we aren't talking about waifus and personality.
Now about Femerlin:
Her support abilities:
- team attack
- team buttery
- team overcharge
- team invincibility
- some strange type of NP drain from enemies
- targeted Arts, Crit and Max HP
- her NP is probably the same with Merlin's, but with two differences:
- her buff is Max HP in opposite of Merlin's heal
- her NP gauge per turn scales with NP level from 5%/turn to 10%/turn, and Merlin's always stays 5%/turn
Her passives are also awesome.
My thoughts about her niche:
She can't be used as DPS or farmer, but her Arts, gauge and overcharge supports in team with Castoria and some Arts DPS can kill everything.
She is also very usable in 90++ quests because of her third skill, which you can use in first turn, because, in opposite of usual NP drain that just removes 1 tick of enemy's buttery, which you can't use in first turn because this buttery is still empty, Avalon's skill is stealing a tick after one turn so it will work.
Result:
I like them both but I'll go for lady A with no regrets.