Well, most of the 'magic' in Teudogar is based on the assumption that if everybody believes in a concept, this belief by itself makes it a strong force, even if the concept is factually untrue and the belief irrational. Historical sources indicate that almost all Teutons, and even most Romans, believed in magic. In my opinion, their belief in the effectiveness of magic made 'magic' actually effective for them: I.e., it's not the curse per se, it's the reaction of a superstitious person to that curse that counts, and that makes this sort of 'magic' a reality in a society where most people were extremely superstitious. (And I think it's almost impossible to grow up in a society without more or less accepting most of the religious or ethical convictions that everybody believes in. If everybody believes in magic, so will you, at least to a certain degree.)
So a person who got cursed would have been very likely to lose hope and confidence, thereby turning the 'curse' into a self-fulfilling prophecy: His belief to be doomed to fail would cause him to be timid and not put 100% effort behind his action, or to give up at the first sign of resistance, thus causing him to actually fail (thereby, by the way, reaffirming everybody's belief in the effectiveness of magic). (Teutons may have been particularly vulnerable to this due to their belief in 'Heil' (good fortune/fate), i.e., that the outcome of one's actions depended just as much on one's good fortune as on one's own efforts.)
However, I agree that the Weaken, Protection, Frighten, and Dominate spells go a great deal beyond what is historically documented and realistic. In reality, Frighten would have worked only if you were a priest (people knew attacking a priest would bring his god's wrath on them). Weaken is basically just another word for Curse, but is too powerful, especially when combined with Curse. And Dominate can't really be justified, nor can Protection be: Increased confidence may be great for helping you act successfully by encouraging you to apply 100% of your skills, but it's difficult to see how it could help you react more effectively to your enemies' attacks. So I may optionally or generally disable the Dominate and Protection spells, and reduce the effectiveness of Bless and Weaken.
As to the potions, these are mainly based on the placebo effect, and therefore, I believe, not that unrealistic: Like in today's clinical studies of new medications, a significant number of patients from the test group, who received sugar-pills without any pharmaceutical ingredients, but who believed they were getting the real medication, will actually show strongly improved health afterwards - caused by nothing more than their own belief in their 'medication's healing power. So I think that Stay-Awake, Love and Virility make some sense as placebos, and Poison, Antidote, Sleep and Berserk make pharmaceutical sense as well (there's a posting somewhere in this Forum about Berserk and the toadstool mushroom). As far as I know, people's knowledge of pharmaceutical herbs has been quite good ever since the Stone Age. But yet again, most of these potions are far too powerful, and I maybe ought to reduce their effectiveness. And I need to make sure that the purely make-believe potions (those without actual pharmaceutical ingredients) will work only if the person receiving them knows what he is receiving and believes in its effectiveness.
Thanks for your idea of disabling magic via the options menu! I think what I'll do is offer a selection 'Magic: realistic/powerful', with 'realistic' reducing most spells'/potions' effectiveness and disabling those that are obviously unrealistic.