I boarded at a private stud barn a while ago that fed two full coffee cans twice a day of sweet feed.
The stallions were QH and they bulked up very well; used part of the energy, gained and stored the rest as fat.
It takes time. You've successfully put some weight on her ribs! That's an accomplishment! The rest will come. If you feel that your process will be efficient over a period of time, keep doing what you are doing. If you are still interested in change, then I would consider switching feed and increasing grain. If she has difficultly digesting she will not get the nutrients she needs from the feed. There is feed especially design for sensitive seniors, even feed designed for seniors without teeth. (LMF Senior comes to mind.) There are options! This feed is much easier to digest and consume. I would also recommend walking her daily . This will help her in so many ways. It may also stimulate hunger because of the activity done. It may also work out stiffness and soreness that may be decreasing her appetite. (I trust you can judge whether walking would benefit. My horse is in severe pain when he is boxed in with no space to move. It is the quickest way to make him lame.)
As I'm sure you know, transfer feed very slowly. She seems to be a very sensitive girl. Research feed as well and the maximum amount to feed/most efficient feed. You could also look into a weight gain supplement, although I'm not sure how well they work. And as far as worming, are you absolutely sure she is worm free? Dewormers didn't cut it at one point in time and I put my critter on Strongid. It worked very well. I just don't want this girl to be over whelmed with supplements! I'd like to see a picture of her, we could watch the progress!