If the Bible "doesn't tell us about all conversations that happened" (which obviously in a lot of cases it doesn't), then all sinners (mankind) can justify ALL sin by using the argument such as, "God said THIS here, but we don't know of all the conversing that took place and he may have changed his mind." (which God has done before)
You are correct, anytime we see an argument from silence, such as Eve added to God's word, we should drop a red flag. Bible study is difficult, but speculation is the mother of false doctrine.
We know for sure two things, what Eve said God said is different from what He actually said to Adam as recorded in scripture.
And two, we know that it was by Adam's transgression that sin entered the world.
So if we assume Eve added, then she sinned, unless that too was part of her being deceived, i.e. she thought that was what God said. So we can spin an endless tale, speculating this and that till the cows come home, or we can try to discern God's message conservatively, asking the question, what is the least God could be saying.
