Besides Yenna's plot involvement in the camp, is there supposed to be a questline for finding her mother? I never really agreed to help her so maybe that's why I don't have anything related to that (and if that's the case how could I initiate such quest afterwards?)
Baldur’s Gate 3 – How to Find Yenna’s Mother
baldurs-gate-3
Related Solutions
Patch 2 added the ability to remove player characters from the party via Withers' Wardrobe. Talk to Withers in camp once you have recruited him:
Introducing Withers’ Wardrobe of Wayward Friends! With this woodworked wonder, you can now dismiss co-op party members and bring your companions back into the fold.
The spell used to summon Scratch is Find Familiar!
Generally speaking, there are two factors at play here: concentration, and spell uniqueness.
The interactions with Concentration are straightforward -- you can't concentrate on two spells, so two spells that summon creatures and require concentration can never exist simultaneously (at least, not from the same caster!).
This means you can't cast "Conjure Woodland Being" multiple times in order to fill the tactical grid with critters.
The other major interaction is Spell Uniqueness -- you can't benefit from the same spell more than once. This is why Scratch disappears -- you've replaced the previous "Find Familiar: Scratch" spell with "Find Familiar: Crab" (or what have you) -- same as if you cast the spell while your current familiar still exists.
I suppose one final variable is duration -- for instance, Spiritual Weapon lasts 10 turns, whereas Animate Dead lasts indefinitely (or until a long rest, at least). If the duration is too short, you'll find it harder to get overlapping summons.
Best Answer
No, you cannot find Yenna's mother. Nor is there a quest to attempt to do so.
The presumed "why" is actually pretty interesting, and it's entirely because of the particular role Yenna plays in the overall plot -- she's there as a schrödinger's trojan horse.
Act 3 Spoilers