#3 is the hardest question to answer. The answer is, yes and no.
I know you really want the details. But it is a bit involved. i'll try to give you one points that i think can answer part of that question.
Engines are designed to burn Gasoline or dessel. Sticking to gas engines for now, gas burns slow compared to HHO. gas engines ignite it fuel up to 50 degrees before TDC in order to time the burn optimally so MOST of the heat is turned into kinetic energy by the time the the piston is going in the downward movement (starting at about 10 degrees ATDC). If you speed up fuel burn by adding to much HHO the power will start working while the piston is still moving upwards and then The HHO is counter productive. that is just one reason you can have to much HHO. so if you can change your timing and the timing advance curve than you add more HHO. Thats the first hurdle
When you're one step ahead of the crowd you're a genius.
When you're two steps ahead, you're a crackpot."