#1. I have no idea, It just works for me. My wideband reads afr's and corrects from there.
#2. From my understanding, A rising rate is installed in the fuel return line after the stock fpr to modify fuel pressure by closing or restricting the fuel return under boost. An adjustable is installed in place of your stock unit
#3. 19lb injectors are stock, at least for the m42 and I believe for the m44 as well. I did not try to run my mustang injectors cause I knew bigger ones work for sure. So I would just play it safe and get some bigger ones like 22's, 24's, or even 30's incase the mustang injectors fail.