Flourish excel should be dosed either daily or every other day, unfortunately once a week would deprive your plants of any source of carbon whether it's co2 or a liquid source. And dosing a "weekly amount" of excel at one time to any tank would result in loss of fish, and beneficial bacteria. It's pretty easy to overdose flourish excel, this is why I recommend a diy co2 system.
As for any kind of air stone, at night plants actually take in oxygen and emit co2 because there's no lighting so if you have something like a Coralife digital timer you can program it so an air pump comes on once the lights go off.
Plantex csm+b = micro nutrients
Potassium nitrate KNO3, monopotassium phosphate, and potassium sulfate = Macro N-P-K nutrients
Magnesium sulfate MgSO4 = Calcium and magnesium are the 2 fastest consumed nutrients in a planted tank.
As for a powerhead, sure you can add one to the tank for water circulation, I always had one in my past planted tanks.