The veteran company GP doesn't affect the rangers which are in the Blackhawk when you make it.

It does, but the veteran icon won't show up until you extract the troops and get them back inside the chopper again.

Probably an engine limitation. The method used for this GP is pretty much an exploit anyway.

How is this GP an exploit? Especially considering not only is there explicit calls in the code that allow you to build units as veteran if a science is unlocked, but also the fact that it was present in the base game.

Speaking of base game, this did not happen with Troop Crawlers in vZH with Red Guard Training.

This is probably because of the trick (Bike logic) that Hunter used to fix Chemsuit + vet company crash bug. And I guess by exploit, he means this trick.

Basically, VeterancyGainCreate doesn't work with chemical suits, so hunter found another way using bike logic, adding the VeterancyGainCreate to riders inside the affected US infantry, to make veteran company work.

The blackhawk bug is caused by the fact that you can't garrison something that's already garrisoned, so the effect only takes place once the rangers exit the blackhawk and the riders can get inside the rangers.

I feel like an idiot for not remembering that.

Basically yes, this unfortunately cannot be fixed due to the aforementioned engine limitations. I would consider it a minor bug however, since at least the rangers have their veterancy.

Couldn't the BlackHawk use dummy Rangers with working veterancy that turn into actual Rangers the moment they properly appear on the battlefield?

much work, little gain