Ok so it seems that basically the U Offset goes from infinitely small value to infinitely large value in the spinner/box but in reality 0-degrees to 360-degrees is mapped to 0-1, which explains why I wasn't getting any feedback in the render... I was using values like 45 and 90 and 270 etc. whereas we need to use a value of 0.25 for 90 degrees, 0.5 for 180 degrees and so on, which is not user-friendly :)
So the first thing I would do would be to have a dedicated section in the CoronaBitmap called something like SphMapRotation mapped from 0-360. This would make it 100% clear how it works.
I would then have a simple Gamma control and you're done. I don't think there's anything else that would be needed that isn't already exposed in the shader.
By the way, maybe it's just me but HDR/EXR seems really really slow to load into the CoronaBitmap and in the material slot, and then to update in the IR viewport. The first time you start the IR, the HDR loads almost instantly and renders fine. But then if you change the offset, it seems to take a long while to recalc before it renders again.
Just my feedback.
EDIT: I can't seem to reproduce my own crash by removing the HDR from env slot. I guess it might have been an isolated thing.