For me, at least, being sensitive as uke doesn't have to do with "blending" as much as feeling what nage is doing to me. It's the same exact thing that I try to do as nage. Some folks might think of it as "listening" to what my partner is doing...
Far be it for me to go against the idea that nage is an "angel" and uke a "devil" but, to me at least more and more over the years, I don't see any difference in the two with said "roles" being artificial labels at best.
As far as the difference between a forward breakfall and a backfall in a technique like iriminage goes, I still think it's about the same difference between a forward roll and a forward breakfall in that, as uke, you just need to stay aware of the circumstance and move accordingly.
Once again, you might want to grab someone with experience who can take you to the point of unbalancing in various methods of iriminage. Some will feel like a backfall will be appropriate while others will feel like a front breakfall will be appropriate. I think you can get to that point of feeling the differences without having to go all the way through the throw itself. One exercise you might want to figure out is: what's the difference between an iriminage that requires a front breakfall and one that can be taken with a back fall?
In one respect, you might be able to picture it as how your own center as uke is interacting with nage's center. Is nage's center above uke's? Or is it below? And so on.
In any case, this has been an interesting thread so far. I'm looking forward to hearing other people's thoughts on this subject.