Most? What says most came from the Waters Below? It says the fountains of the deep opened up AND the windows in the firmament were opened. Doesn't say how much came from which, far as i know.
Where do you find support for your claim, here? God of the gaps, noted.
The firmament I don't yet know the explanation being an unfathomable. So one out of many humdreds, so to speak, is fine, I'm sure you'll accept I don't know everything in the bible.
And this, then, is why i call your stance based on bullshit.
The book is very clear that the firmament is a solid dome.
The Earth is lke an inside-out snow-globe, a bubble of air that has a flat bottom (the Water Below) and a dome keeping the water out above. Except when God wants rain, and opens the windows in the firmament, to let water fall through.
But you know that's not accurate to the real world. So you edit scripture as necessary, making parts literal when you think you can get away with it, and allegory when you're forced to, so that the Bble doesn't egregiously violate reality as you accept it.