Not to butt in, but I also have heard that corn is higher in starch than many grains and also hard for horses to digest. Not that you would be harming a horse by giving them corn (unless they were Cushing's), but in my mind there are better substitutes I would rather feed.
Grass: as far as I can tell, although what you say is true for most horses, a truly hard keeper may need pounds of added food in addition to hay, vitamins, and probiotics. And as you said, too much grain is risking colic and other problems. So complete or senior feed is a good alternative.
I used to feed COB in small amounts to horses that were not hard keepers. They were always healthy and a good weight on just a little grain with vitamins as long as they had enough good hay. But if the OP's horse is like mine, you just can't get her to eat enough hay. Hay pellets are not any more attractive to her, and she dislikes beet pulp wet or dry. So for her, senior feed is the answer since you can feed it in large amounts to supplement the hay calories. I also love feeding corn oil as it is easily digestible and adds easy calories.