I would like some help with merging, SI and stuff

I have some duplicates I'd like to merge but i don't know if that is the best thing for them. Nowi: both +spd -res one fully built and the other not. I think i want to merge them considering the only thing nowi can give for SI is her breath because i already have a few oboros for threaten res and i have a 4* star nowi with bad iv's i could always promote for the breath to inherit. Robin male: +atk -hp and +def -hp. The +def has a load of sp but the +atk has the better iv so i don't know if i should give the one with the sp the skills i want to inherit and then learn them and then merge into the +atk or to build the +atk and just keep both. Roy: I have +res -hp with a lot of sp but no build and a +hp -res (irony lol) that doesn't have any sp or stuff yet. I could merge him into the +res because that's a better iv but i could also use him as SI for male robin for TA. I could alternatively upgrade a 3* selena or 4* cordelia(bad iv) for TA for male robin

My last point is about what kind of build i should do for nino, soren and sonya Nino is fully build already with darting blow, moonbow desperation, hone attack drag back and -hp +spd as iv so she's pretty much full offensive and i want to give LnD3 sometime to her as well My soren is +atk -def but he's still using the Base kit and wanted to give him fury to make sure i have unit that can check reinhardt before i got my sonya. I could also make him a full offensive green mage but i don't know if I should do that considering i already got nino or if i should try a different offensive build on him Sonya is unfortunately -res but she's +spd at least so she's not completely ruined. I wanted to make her the reinhardt counter with her neutral res of 32 but now it's lowered to 29 so i thought about giving fury to her but soren has the same res then so it would give the same results in terms of res.

Soooo that was a long and also my first post on reddit lol, anybody who wants to help and bothered to read through that huge wall of text I'd really appreciate it.