I told you already, what can be discussed with 100% certainty is how to tackle the Bible - what it says, what it means, the author(s) purposes, lessons, themes, etc.
As long as the debaters agree on which translation of the Bible, yes, the Bible may be discussed with certainty. The point I'm trying to make is "if the Bible says something, that statement is
not automatically true." If you are discussing "what does the Bible say?" then, as you have said, we
can discuss that with relative certainty.
These we can scrutinize and have a decent debate over so instead of ending your arguments with blanket "how can we know anything for sure" statements it's far more effect for you to find some arguments/scripture/citations that potentially support your what-if theories.
Without concrete proof of the afterlife, we cannot know. If you're asking from a quote from the Mayahana or some other holy document that says "babies have a special Hell", we probably won't find one. Ancient, allegedly-holy documents don't make your argument any more true (unless your statement is something along the lines of "When was John Smith born?"). Otherwise I could claim that any document is inspired by God.
By only discussing the Bible, you are ignoring the Bhagavad Gita, the Mayahana, the Torah, the Quaran, and more. If you wish to only discuss the Christian God, then yes, the Bible says that babies go to heaven. It's clear that you know a lot about the Bible and I know very little. That is why I ask questions, even though the answer might be obvious to people like you.
Of course, if this topic is only to debate "free will and the
Christian God," I'll gladly remove myself from the debate.
Win by luck, lose by skill.