Shortly after I finished reading UFF 16, the ending seems pretty self-evident, and it seems that everyone here has pretty good ideas as well. (However, if I'm all wrong feel free to laugh in my face and toss monkey poop my way)
I think the idea of Ben and Reed can be synthesized this way:
First, if Reed has a pliable bacterial stack that draws all of his nutrients out of the air, he is essentially eating through the air. The thing is, it's NEVER been specified what the limitations of that system is. Does he need to breath just oxygen or does he synthesize gases --- nitrogen, CO2, oxygen, hydrogen --- into the complex forms his body needs? That could make sesne, since as far as I know carbohydrates, amino acids and other nutritional needs are made out of those core elements.
Even Sue helps out by saying in # 15, "Not a whole lot of complex particles. Not a lot of hot spots. Textbook late entropic system." Maybe I'm reading too much into this, but if a universe is decaying, doesn't that mean that life and matter on it are decaying as well? Meaning that Reed would be able to "eat" the air of that system just as easily as he would back in our universe?
As for Ben, Sue remarks that she's absolutely unsure as to how Ben is even capable of breathing. "He weighs like, half a ton, how does he inflate his lungs?" There isn't any more explanation than that but what if Ben doesn't even need to breathe? What if his blood consumes something else to feed his organs? What if Ben breathes through his petridermis?
The thing is that while the N-Zone has already been explored as a catalyst for genetic and quantum change, it has only revealed to us the "changed forms" of the Fantastic within the context of our own universe. Which means we still don't know how their powers work within the N-Zone and the very unpredictability of that is made even more definite by Johnny's condition.