As best I can recall (as someone who read all the books and played all the games), Ciri never underwent the mutations in either the books or the game. In one of the endings of W3, she becomes a “Witcher” but imo it was always implied that she was just going to use her own ludicrously overpowered abilities and didn’t really need the potions etc.
Yeh. If i understand matters witchers are one of the less powerful magical beings in the setting and are restricted by the high mortality rate of their profession and their very limited and specific skill set.
Geralt is an absolute monster of a combatant, but in the end he’s still brought down by a mob of normal humans (in the books).
He is incredibly powerfull, but a mage specializing in combat magic wipes the floor with him.
Overall witchers seem to be jack of all trades, some useful magic, some very good close combat skills, some useful alchemic skills. But a witcher won’t outmagic a mage, win a duel against some sword saint level fighter or brew better shit than some alchemist studying the craft for decades(granted, said alchemist wouldn’t survive witcher brews).