Do feelings of lust and incidental erection while reading a fatwa or performing istibra' (cleansing after urinating) constitute a lack of adab (manners/respect) and lead to apostasy from Islam, and what is the effect of that on the marriage contract? And do uttering statements and performing actions that lead to apostasy from Islam in childhood, despite current adherence, affect the marriage contract?

You are not to be blamed for what happened to you, because you did not intend it. The proof for this is the Almighty's saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended." A Muslim must guard his tongue, as stated in the Hadith (definition): "Indeed, a servant may utter a word that displeases Allah Almighty without attaching any importance to it, and due to it, he falls into Hell." Your commitment and adherence to prayer thereafter counts as repentance, and what you mentioned has no effect on your marriage, so long as it occurred before the marriage and you repented from it.