The Phoenix worm shop probably suggests that you don't feed the larvae to keep the process simple. It may also slow down the rate at which they go through the larval stages. Some people who feed small critters don't want the larvae to be large and with food they grow quickly as you know.
Your observation about the cow manure makes sense. I've been observing that my larvae process food much better when it's fairly wet.
I believe that most studies using BSF to process manure state that they do much better on fresh waste. Maybe bacteria consumes most of the nutrients in older manure.
I'm learning that BSF larvae can prevent bad smells from developing, but they can't eliminate existing odors. I learned this the hard way with a batch of dead fish that had been floating in a friend's pond for several days. On the other hand, I've been adding recently dead fish to my unit with no odor problems.
I don't know what the minimum amount of food necessary to produce BSFL would be. I think less food would probably result in slower development and smaller adult size. I would guess that the survival rate would be lessened as well.
I think in a typical open BSF composting system you would have trouble trying to raise larvae on smaller rations. My experience suggests that the colony will adapt to the amount of food added to the unit. I'm not sure, but I believe the female BSF are more attracted to a system that has excess food. My theory is that if you overload a BSF unit with food, that food will begin to smell since the colony can't process it quickly enough. The increased odor level will then work as a stronger attractant to BSF females resulting in the colony expanding. I'm thinking this may work in reverse so that if you withhold food you will get less eggs deposited, and therefore your total weight of larvae will be reduced to match the food quantity.
On the other hand, I've been adding recently dead fish to my unit with no odor problems. 
