I am not sure if I understand your request correctly, but if so, then I think you can achieve what you want using the following setup of operators:
the photographic exposure takes the exposure from the camera and uses that as the "base" (what the rendering is initiated with)
then the simple exposure decides about the additional exposure adjustment
What I understand is that you would like to take advantage of the fact that Corona considers the "initial" exposure for determining what GI should look like. This leads to the following effect:
https://support.chaos.com/hc/en-us/articles/4528530828689-Exposure-changes-when-I-start-rendering-If you want to get faster rendering / less noise at the cost of some bias, instead of using that "trick" I would recommend adjusting the max sample intensity value in Render Setup > Performance. Lowering this value will make GI darker and the scene will generally render faster. The default value is 20 and values around 10-5 should still give you some visible GI. Lower values will make GI almost invisible. The value of 0 removes any sample clamping so your GI will be brighter and you will get fireflies. :)