A Hurri IIa is a Hurr I with a more-reliable engine (which doesn't matter in AH terms). A Hurri IIb is a Hurri I with bomb racks (not applicable to this battle, and slightly slower, by the way) and two more 303's (not that big a deal). Thus Hurri IIa and IIb are about the same as a I. A IIc is not at all the same as a I, is a poor fit for what was there, and would be hugely unblancing in this fight.
As a person who signed up for Hurri I's in this event, I'm not that worried about it. 8 303's that spray out a huge cloud of projectiles might be quite excellent weapons against Japanese planes that are the most flammable in the game. Regardless, I want to fly, fight, and win my victories in the more-historical Hurri I!
Now, one could argue that D3A's are a more accurate subsitution than a6m2's -- but we figured no one would sign up to fly d3a's as a fighter substitution, we'd get nothing but grief, and we don't have the luxury of trying registration then changing it if it doesn't work. (Of course, if we had the more-realistic d3a as a main Japanese fighter, we should have a more-realistic ratio, like 5:1 Japanese odds or something, and again very likely get no one signing up.) The registration process and timeline is more like maneuvering a battleship than like maneuvering a PT boat. You can't try it out, stop it if things aren't working, change things around radically, restart it, and have things proceed on plan. You can't slolom around with it. You have to pick a direction and go for it.
For the Japanese plane set, the a6m5 is in there because the Brewster model we have is perhaps better than the Brewster model that was there; and to even things out some the other way, the allies have some P-40E's, which weren't there early on.
The c.202 isn't a good fit for any Japanese plane in this fight. It is enormously faster and handles well at very high speeds, which is radically different than the Japanese plane set. Armament is only one of many dimensions to consider.
Scenarios aim for realism and playability, and those features often pull in opposite directions that require tradeoffs. Not everyone is going to agree with the tradeoffs that are made, no matter what those tradeoffs are. The scenario team picks things based on those goals and its experience in running scenarios. It might not be perfect, but it's our best guess at what best balances realism and playability.
