I do most of my battles with autocalc (the battles usually take too much time, I have a finite time to play!), and have several 3 chevron line infantry which got there after 4-5 major battles. Autocalc does seem to give more XP to units than manual battle.

I'm still puzzled about this reinforcement bug that you are talking about though. I replenish my units, and they keep their chevrons just fine. Heck, they better do that, replenishing a 3 chevron unit is 2x or 3x more expensive than replenishing a similarly depleted no-chevron unit!