He would probably become most physically powerful mortal. Based on his ability to intuitively create illusions with no training and just some help from crystal given to him, Gearalt could probably get as powerful as Yennefer if he studied magic since childhood, and be somewhere around an average sorcerer if he started studying late in his life. If you factor in Geralt's inhuman speed and reaction time, durability, potions, and experience, he could probably outmatch Vilgefortz.
Just imagine Geralt using his illusion skills to create multiple images of himself to distract opponents or using his knowledge to develop signs further and making them more powerful.
She is not. She rejected learned magic of elements and all she has are magical abilities that she inherited from her ancestors. Ciri can't cast spells any more, and her ability to control giftes of elder blood is imperfect.
Yeah. Elves are like upper middle class and upper class welfare kids that lack ability to compromise and adapt, can't come up with anything of their own and either ask parents for stuff or steal it from others. For Elves, their parents were Elven deities and a small community of ancient elven wizards. Once both of these were gone, Elves had to hide in the mountains and forests while barely getting by on what they can poach or steal.
Meanwhile, inferior humans developed agriculture, domesticated various animals, defensive architecture, alchemy, machines like siege engines, robust postal and courier system, dams, mining industry to rival dwarves and gnomes, and learned how to use magic without help from deities, being less magically inclined and having shorter lifespans than elves.
High Vampires seem to be pretty unkillable. Vilgefortz completely burned Regis with flame spell that left only a glass like husk, but Sapkowski still implied that Regis wasn't out completely both in interviews and Regis' ghost not being present at the end of the last book.
They did almost all monsters that had description in the books or Slavic myths. From the top of my head, there was still Oczoglów, actual Ehinosp which should look like porcupine/hedgehog on steroids instead of a plant, various werefolk like wererats and werecats, three monsters of unknown species that Geralt encountered when visiting Toussaint in the books, and Dębostwór which is sort of like an Ent from LotR but not intelligent.