I buy my corn oil from Wal-Mart, just the biggest bulk container they have. I do know someone who tried corn oil on their pony and it didn't work. On closer inspection of the package, it was fat free corn oil
! I didn't know that was physically possible, and I've never found fat free corn oil myself, but I'd check the packages to be sure. If it's cheaper in your area, regular veggie oil works, too. I'm not sure about the beet pulp, I've never looked for it before.
I did a little research on the hunter bump, and it looks like it can
be caused from an injury, is common to jumping horses (hence the name) but any breed or discipline can be affected. Some horses find it painful, others don't. Basically, no one really know exactly what it is, lol. This is based on 10 minutes of Googling, I had never heard of this before. Chiropractic adjustment seems to be the treatment of choice if it bothers the horse. Your mare's hunter bump looks more prominent than most of the pictures I've found, but I'll bet that's because of her weight. The angle of the "before" pic is tricky to see, but I don't see it nearly as prominently in her "heavy" picture.
To get a better idea of the state of her teeth, take your left hand and hold the bridge of your mare's nose, midway between her eyes and her nostrils. Take your right hand and gently grip her lower jaw and try to wiggle it anck and forth, and then side to side. If she has any ridges, hooks, etc, her jaw won't wiggle freely. This may not work if she clenches her mouth, but it can be a decent indicator. I've seen horses in a bad way dentally that didn't drop feed. Also watch when you are riding to see if she's evading the bit, this can also be a potential indicator of mouth pain or dental issues. If it's been a year, she is due for another checkup. I think the recommended time between checkups is 6 months to a year, just like people.