I just got HAWX because a retailer here is going out of business (no pricedrop yet outside of the 50% blanket discount that one gave)...
There is more than one speed, hold the triggers down to go faster or slower. It's not a large difference but it's more than one speed. I find slowing down useful for getting more time to shoot at ground targets.
Failing the first mission gotta be fairly pathetic, I ran it on Hard the first time and then on Expert, no real difficulty. The reduced ammo of Expert makes no difference since you won't use nearly 100 missiles and the increased damage doesn't matter if you've got the tiniest bit of skill and just avoid getting hit (it's trivially easy anyway and that's without safeties off). I did run into the ground once when I tried to gun a ground target to death, after that I just learned not to do that and went with missiles instead.