From: Bratislava, Slovakia
Hi, welcome aboard.
As to your questions.
A) Base building - I would say that the base building is about right for the allies - they were able to pour men and materiel onto any field and make it a base in no time - few days for fighter strip (lvl 1/2 AF), a week or two for medium bomber base (lvl 4-5 AF) and month for heavy bomber base with hardstands, revetments and all (lvl 5-7 AF), or throw around an airfield complex like Torokina/Nadzab/etc. Japan has the going much difficult because of lack of mechanized force (bulldozers, etc). This is reflected in the game.
B) Durability is not the only factor - first you have hit the target and then to defeat armor. Hitting nimble Jap fighters is not that easy, but there are ways. So basically you can shoot down (destroy) any 0 armor aircraft with light MGs (+/-7,7mm, those with armor 1 would be damaged at most. For Armor 1 you need heavy MGs (+/-12,7mm) to have a chance to kill (destroy), yet even that would cause a majority of allied aircraft damaged (high DUR), while the Jap planes would mostly be destroyed (low DUR). Using cannons (+/-20mm) would most probably result in destroyed opponent.
So you need light weapons to destroy airplanes without armor (Ki-43-Ic 0 armor, 23 DUR), heavier weapons to destroy planes with armor (P-40E 1 armor, 29 DUR), and even then you would need a hell of a luck to down allied heavy bomber (B-24D armor 1, but DUR about 60).
Actually some people consider allied heavy bombers as best fighters in their inventory - lot of defensive fire shooting down Jap fighters, hard to kill and they carry bombs!
C) Japan has about the correct number of merchant shipping than IRL. There are two things that are altering the RL picture:
- IRL a number of merchant shipping was used to carry things around, the amount of resources/supplies to be transported in the game seems to be a little low for that
- IRL the Navy and Army got their separate ships assigned - that were not able to be used by the other, nor by carrying general freight around. So ships tended to carry just troops/supply there and empty back, while in the game you can carry troops/supply there and resources back.
For better feeling of the game, I can advise you to check DaBabes-C scenarios. DBB as a whole has taken airplanes and their stats, AA stats and ToE changes (better modeled planes, AA that actually works deadly, ToE revision). -C scenarios has reduced cargo capacity for all ships by about 1/3rd.