No offense, but often your 'logic' is just baseless/pointless. What difference does it make if Peter is a male or not? more often than not, men tend to stray further from their childhood beliefs than do women, not vice versa. And, may I add: men are typically the ones more prone to party hard/'do teenage things' than women. Typically, not always.
As for myself, I have always thought that Lewis intended Susan to be viewed as a person who has never solidified their belief in Christ. That is, their 'Christianity' is only on the surface and a decision has never really been made. Whether or not this means that SUsan would eventually come back to her faith and actually believe in Narnia/Aslan is something Lewis did not tell us, and I don't think we're really meant to know. Does she remain skeptical? Does she realize her siblings were right and come back to her childhood faith and really believe? Who knows. It's one aspect of Lewis' writing that has always aggravated me. He leaves so much to the imagination it's frustrating.