The advantage on stealth shouldn’t effect the throw into a square as the square cannot be stealth attacked as the square cannot react to the attack anyway, but due to that holds a flat AC of 10. (You can’t surprise the ground, you can only surprise something that can be surprised)
Also if the square is being attack wouldn’t it follow that the person in said square can make a ‘reaction’ save(eg, Str, Dex and so on) to avoid it or reduce the effects of the attack on the square, as one does with spells to reduce the damage or effect?
If not wouldn’t it be possible to build a grenadier character that only attacks DC 10 to hit anything.