Getting exrtotiff to work gave me more headache than pbrt itself! It happened that my VC was a rather older version and the exe file couldn't run on my machine. I ended up using a friend's machine and the problem is still there to resolve. I didn't have much time to look into exrtotiff to see if I can save the tiff files with different exposure parameters, some of my results do look closer to what was expected by changing that factor.
Although it took me more than what I expected, the homework was pretty straight forward. Some factors that took me longer to get familiar with were: For area ligths I preferred to work with object coordinate rather than camera. The size of the area ligths are very important and can change the shading dramatically.