they would make the most sense, but at the same time thinking on it could we even use the magic they have? Vicoria is a succubus and the witch is.... special in ways explaining would spoil one of the biggest plot twists I semi saw coming but still loved because it was executed FLAWLESSLY, and the nameless is straight up just a eldritch abomination that's painfully hot lmao. so considering that maybe the most likely person to teach us magic would either be not a person but some random spell books we can find across the world like you said or an entirely new NPC we have yet to meet whose either important and we befriend during one of our chaos filled adventures so they decide to teach us magic as thanks or one of us seeks the other and asks to be taught or to teach us because either we want to learn it or they see great potential in us.
but if we do learn from them I feel like the witch would teach us buffs for ourselves and allies and maybe some magic to enhance our potions effects like making a strength potion boost 15 points instead of 10, or causing a lust potion to linger as like lusty mana and become like a semi-permanent (long as you got mana) lust powder.
Vicoria I don't think would teach us actual defense magic like a barrier or anything physical but more like how to toughen our mind to resist other succubus/incubus powers, or how to wield the powers of a succubus/incubus ourselves to make fights end quicker and potential fun start faster.
nameless I think would be pure debuff spells since we don't know if he has any physical weapons or offensive magic that doesn't include corruption or mind control. actually maybe he could teach us a spell to make lures useless and just mind control them into following us home lol.
and more for Nameless, his magic causing a bad end is actually a fucking sick concept! like the more we learn or use it the more something changes, maybe it would slowly corrupt us and make us appear more demonlike or maybe more like the nameless just also kinda human since we know so little about him we don't know if he was always that way or not lmao.
also off topic, maybe the spell books could also have lore involved? like some of the spells we don't learn directly from someone else explains certain things they don't exactly care to explain? like what the Nameless' limit's are and how his powers work, or how the spoilers were born and how long they've been actually been around! it also could finally help me decide whether or not I can trust nameless..... he so hot it's hard not to but I'm scared my friends and the other hot men I sleep with everyday cherish with all my heart could get hurt :(