a) Would it do enough damage to justify cost of building and maintaining?
Not to Germany, who only has 5 convoy damage to do and 3-5 factories. Perhaps to Japan.
b) How would axis respond by about turn 5 with 15 subs/10 bombers operating?
Annihilate India then Moscow before you can get set up. They just ignore you while they grab threshold income (70).
c) Would you be better to target one axis member or disperse individual subs across board?
In most cases, one (though I argue some splitting by USA is both desirable and necessary in every game)
Your plan is pretty solid, though as a sole-economic warfare thing, it doesn’t work well for a couple reasons. Subs cannot defend themselves, either alone or in number. YG and the group discussed a sub-dispersion strategy that forces the DDs to also spread out, but Germany can just ignore them and so can Italy and Japan until they are standing on their 3 money squares (6, 19, Adriatic).
The only way you can contest control of those key economic squares is by overwhelming force–and subs and bombers help provide that. However, if you only do subs and bombers, you are adding very little to the DEFENSIVE power of your fleets on the water, which means they can just be ignored, too.
What is necessary is to add them with more diversity, so that you do have some subs, and some bombers, but that your fleet has enough carrier aviation and destroyers as causalities that, when combined with the subs and bombers on the offense, it is truly intimidating on BOTH offense and defense.
Great post, ty for the discussion.