What has worked for others may not work for you. It seems each cell design has its own characteristics and you will need to experiment.
Efficiency seems best if the voltage is kept low. Many say the best voltage is between 1.5-2 volts, but yours may NOT work at 2 volts or below. This may vary with the type of electrolyte used, plate shape, spacing, water purity, electrolyte purity, and a host of other factors.
Large plate size may not be the best solution. My plates are 1/2" x 5". If the electrolyte becomes discolored, it usually means the plates are being eaten away. A greater number of smaller cells may give the best electrode life.
The efficiency increases with temperature. There MUST be heat generated. Energy is used in the process because ions in the water need to move to carry electricity, and this movement heats up the water.
Amount of gas is NOT proportional to the current. Engines will see a gain in fuel economy and reduced emissions will "as little as 10 amps" for a generator. This does NOT mean the goal is a 10 amp generator; it means we want an efficient generator under 10 amps. Don't do as some have, and just add electrolyte until it reaches 10 amps.
Don't use windshield washer fluid, ethylene glycol, or methanol/ethanol meant as a gas tank additive for the hydrogen generator antifreeze agent. They all contain anti-corrosion inhibitors that will break down your electrolyte.
The small blue aquarium bubblers have a binder that is attacked by weak acid and will crumble easily.
Apparently not. I almost ordered some graphite flat bars, but I saw a number or HHO experimenters, and chemistry pages on the net that say the carbon flakes of as sort of a dust. The chemistry people suggest platinum (yeah, right).
I guess we will have to use stainless until platinum prices drop to $2 an ounce.