I run zone/flex at Virginia union in Knight D2. I have a pretty decent amount of success with this team. Normally, I do not play 3-2 because of the rebounding disadvantage. This season, however, I played a lot more 3-2 because I had elite rebounders. Both bigs were 99-100 for rebounding and even though their ath was only high 50s- low 60s they were both very productive on the boards. My SF was a guard with only 20s rebounding and this didn't hurt me at all last season. I think you can get away with the low rebounding SF as long as you have great rebounders at the 4 and 5, but it really needs to be a priority for your bigs.
As far as being interchangeable at the 4 and 5, I will switch my guys depending on match ups. However, I would really prefer to have the better passing big playing PF, especially against the press. In the past I looked at recruiting bigs, but now I am trying to focus more on having at least one of the two a guy that can handle the ball a little and pass a little. This is more for offensive purposes, so it may be a little off topic.
As for the equation for rebounding, I don't know waty it is exactly or how it is weighted, but based on the last season and comparing that to my other seasons I think by far the most important factor is rebounding itself. I have had rebounders with athleticism in the mid 70s and rebounding in the mid 80s and they didn't produce as well as the 60 ath and 100 rebounding.