To answer your points:
1. The basic idea to separate US Army (here with US flag) and USMC was that both use different equipment, or different versions and/or introduced theirs at different points in time. As this is played from the USMC side, I didn't want to give the opportunity to have a core mixed of USMC, army, army air force or even navy units.
So there's certain stuff you should not be able to buy, since the USMC never used it. But many of the ops depicted were combined between several branches, so somehow army, navy had to be kept in.
So the core is restricted to USMC equipment, the rest is aux, and to achieve that, the USMC is basically made a separate "nation" with a new flag, own unit set etc.
You can bring up the purchase screen for allied nations specified in a scenario, so for US (=army) too, but if it is impossible too buy them all works like it should.
2. Re length, too much moving: I agree somewhat, but it was also a bit of a dilemma. The main problem is map size + mostly difficult terrain. The latter is part of the challenge in the Pacific, so the other way to reduce length/movement would be to make maps smaller.
I fully understand that in some scns people may think it is too long moving etc, but if I reduce map sizes significantly for example for Saipan or Guam I was afraid things would get too easy. Because then you'd probably "storm" over the defenses in no time, but difficult moving, and slowly reducing enemy positions played certainly a major role in the PacWar, and I wanted to capture that.
In short, this is not intended to be so fast-paced, I only hope that I didn't overdo it, and made it too tedious. I have to warn you a bit though - the last scn on Okinawa is really huge and long (maybe too big), and you have to locate the main defenses first. I even thought to break it down in two sncs, but then left it in one. It would probably worth to think over this again, but redoing all the later scns would be quite some work.
Personally I also found some of the vanilla scns a bit too smallish, but yeah - there's certainly a downside when you "grow" them too large, more movement, more time-consuming.
I don't know so much about their tactics. But simulating a banzai-attack with a 1-ammo unit that CAN finish off a weak enemy unit but is out of ammo and thus doomed against stronger units seems very logical to me.
