[Evil] subtype allows you to qualify for Evil things but doesn't require you to be Evil, correct?
Yep. You count as evil, but still count as what your alignment is as well.
Fore reference (
http://www.d20srd.org/srd/typesSubtypes.htm#evilSubtype or
http://www.wizards.com/default.asp?x=dnd/glossary&term=Glossary_dnd_evilsubtype&alpha=E). Do prerequisites count as effects? If so, a Good (Evil subtype) Malconvoker would be in a confusing state. There are four possibilities, as far as I can tell:
a) Assuming that Good and Evil are mutually exclusive qualities that one can have, and that the alignment subtypes treat you as both: He counts as both Evil and not Evil, so he both qualifies and is disqualified from his PrC (and the feat). While paradoxical class features are interesting to think about, I don't think they're very practical.
b) Assuming that Good and Evil are qualities that one can have that are not mutually exclusive: He counts as both Evil and Good. That will qualify him for anything that requires an Evil alignment, but will disqualify him for anything that requires a non-Evil alignment (since he's Evil). Ditto qualifying for Good and disqualifying for non-Good. Even if he's Neutral instead of Good, it's the same problem since Neutral isn't just "not good or evil", but a distinct alignment in its own right.
c) Assuming that Good and Evil are mutually exclusive qualities that one can have, and that the alignment subtypes make you react as though the worst one in a given situation: He counts as Evil or Good, whichever is worse, thus disqualifying him from both feat and PrC. Most inline with the intent implied by the wording of the alignment subtypes, I think.
d) Assuming that Good and Evil are mutually exclusive qualities that one can have, and that the alignment subtypes make you react as though the best one in a given situation: He counts as Evil or Good, whichever is better, thus allowing him to qualify for both feat and PrC. Least inline with the intent implied by the wording of the alignment subtypes, I think.