I would try alfalfa pellets, or alfalfa\timothy mix pellets, to help supplement the senior feed. I am going to assume that the senior feed is a complete feed? That would explain why they would be trying to give her a little more, than say, a normal feed, that is not designed for complete feeding.
Again, though, I would try supplementing the senior feed, or mixing the alfalfa pellets with the feed.
If you are going to just supplement with the soaked cubes, you may need to up the amount she is getting; a 'handful' doesn't sound like a whole lot.
I would try the beet pulp, as well; I know here, there are beet pulp pellets (so much easier to feet than the flakes, if I had to feed them!); they can be fed soaked or unsoaked.
Considering she doesn't have good back teeth, I would try soaking ALL of what you feed her, though...even the senior feed may not be benefitting her much, since they use the back teeth to grind everything with.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."