Basic wattage and volt conversion when using solar panel
If you short circuit the panel the output voltage is zero, so that's not a usable indication for maximum output. The 5.2 W is not correct, it's just the product of the open voltage and the short circuit current, but you can't have both at the same time. The output voltage will decrease as current increases, and you'll never have 5.2 W.  

A linear regulator will draw the same current from the panel as its load. So if the panel gives you 7 V at 400 mA, using a linear 5 V regulator will still only give 400 mA output. The difference in power: 2.8W (7 V x 400 mA) - 2 W (5 V x 400 mA) = 0.8 W will be dissipated in the regulator. You would need an LDO regulator (Low Drop-Out) if the input voltage comes close to the 5 V out.

A switching regulator is different, and can indeed have a higher output current at a lower voltage. The conversion isn't 100 % efficient, so the 2.8 W in will not give you 2.8 W out, but probably 2.6 W. Then you have 520 mA out for 400 mA in.  

Connecting your devices you have to add their resp. currents. Should be less than what the power supply can deliver, the rest will indeed remain unused.