Help on MPPT for solar panels
First question : yes, a buck converter will increase the current delivered to the load as it reduces the voltage. (minus a small percentage loss)

Second question : if the output must be a constant 5V, and you always want to get the maximum power available from the panel, you must continually vary the load impedance to match the power available. So your load should ONLY be 2.5 ohms when the cell is actually delivering 10W. When a little haze reduces the cell's power to 5W (1 amp) the load must be 5 ohms, and so on. Otherwise the voltage delivered will vary from the desired 5V.

Most MPPT systems vary the load impedance by varying the charging current to a battery (or into a grid-tie inverter) If you don't have some sort of variable load (storage system, variable speed fan or even a heater!) you can't really implement MPPT.