Re: fall into or onto
A stone may be dropped into a body of water, because it will sink rapidly through the water. Things like rain or snow or leaves fall onto the water. Yes, eventually what's in that rain will spread throughout the surface water, but it doesn't plunge down into the water.
That said, I wouldn't think the sentece was horrifying if it said "into." I'd just use "onto" myself.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.