Liz, I wouldn't even know how to advise because I am clueless myself. However, the only thing I do know is that you don't know if something works until you try it. So you could try the longer nap, with the longer A before bed and see if that works for you. If the shorter nights are ok, then it would be worth a shot. But if you want a longer night, maybe you could push the morning A more and have a really short A before bed if J isn't handling it well.
As for me, what I am worried about is that my LO's needs are less than 12.5. A month ago, that is what they were and now that he is almost 18 months and their needs supposedly drop more, I am worried that I am expecting too much with the 12.5. I don't quite know what is the right combo for him.