well to know what is the exact ideal temperature, we would need a study that analyses the solubility of mescaline and other alks in water at different temperatures, as well as the stability of these compounds in these temperatures, and I dont think this exists.
But yeah, the point is, stay under the boiling point, this has been shown by people here to be good...
You can use a gas stove anyways.. You can either improvise some metal support that holds the pot farther away from the fire, which shouldnt be too hard, and/or you can, as you said, alternate between having the fire on and off.. the disadvantage of this last option is of course that you have to be around and paying attention, but I dont see why you couldnt do it, as long as you dont let it heat too much and start boiling.