Why all particles have same influence on osmosis
When you dissolve something in water, you build a solvation shell around this - so each ion (in the case of salts) gets surrounded by a number of water molecules. For more details read this Wikipedia article.
The concentration change in free water molecules is usually not taken into account since it can be neglected due to the vast number of "unbound" molecules. So if you have one litre of water, it contains approximately 56 moles of water molecules. If you dissolve 1 mole of sodium chloride in this water, and take in account that this takes quite some water to get solvated, there is ways more water available. So the water concentration is almost constant (especially when you work at physiological osmosis condition).