a) Perdo Corpus Base 30 - kill a person. The manner in which it is done is completely up to the caster correct? It doesn't have to be "he just dies" - the example give is that clenching grasping hand motion ... What about something around a friend brain? I do not have to add Mentum correct?
b) Ignoring wound penalties - "Incapacitated" is a wound penalty and would be ignored correct?
c) Twisting the tongue equivalent - if I would want to create a version that would ... lets say, twist something else causing death - that really is just a flavour for yet another Perdo Corpus spell? Like a part-targeted spell to destroy the mind (physically, not the memories).
Yes, you can choose how Perdo Corpus spells inflict their damage. This is made clear by some of the example spells such as "Evisceration of a Thousand Knives" (GotF) and "Punishment of Marsayas" (AM). Since you'd be targetting the brain physically, Mentem would not be required. My only real concern with brain-targeting spells is that I'm not quite sure the brain was a vital organ in ME (I'll have to check A&A and get back to you...)
Edit: According to A&A, the brain is not only an important phlegmatic organ, but also the seat of the Sensitive Faculty and this does qualify as a principle organ the destruction of which is incapacitating or fatal.
IMO, No. wound penalties are penalties applied to actions when your character is wounded. Incapacitatation prevents you from acting.
Yes, I'd say so. Generally you use the net effect when calculating the spell rather than the action the spell performs. For example "Twist the Tongue" is designed to prevent the target from speaking clearly so it's a Base 15 spell - Cripple a limb making it unusable. Modified to +2 Voice range and +1 Part target for a total level of 30. If I wanted to design a spell to kill someone by twisting part of their anatomy, I'd need to use Base 30 +2 Voice for a total level of 40.
What Gremlin44 said. I'd only note that I'd consider any mind-affecting spell to be Mentem; the brain in ME hasn't got much to do with the mind, I think it's about cooling the body or something... So a spell that stops a person thinking would be PeMe, but a PeCo can just cause a person to drop dead, which is totally equivalent (perhaps severing the connection between the body and the soul?).