Don't Icebreaker and Invective ammo cache's reset on respawn due to death or new round/match start? If so, then the ammo regen will start over each round if players have to rely on those 2 weapons for special. Of course, the team or player(s) who manage to pick an ammo crate will have the ammo carry over if have don't have IB or Invective equipped and they don't use it all.
I think if you keep them on, end the round and switch, you will have some ammo for the weapon you switched too, but could be wrong. I know it's that way in PVE at least.
I think you might be right, but it's been so long since I used either, I couldn't confirm it myself right now.