Hi. I think you're giving yourself unnecessary troubles. Here's some things that'll make life easier:
1) Don't use the exposure control in Max's environment panel. Use only Coronas exposure (panel or camera).
2) Don't mix sun and HDRI. It's not "wrong", but it does require more knowledge and skills, and is harder to make "physically correct". Use a HDRI only or a sun + corona sky.
3) Your HDRI seems to be set up wrong in the bitmap loader. I guess that "use real world scale" is activated in your preferences, giving some headache in max - your bitmap is to 1cm X 1cm in "size", instead of 1 x 1 "tile". To change this, you need to first select "map" mode, uncheck "real world", and the set it back to "environment" and "spherical".
Hope it helps