Do you have access to a mare&foal feed? She is a senior but she's also lactating and needs a higher protein content, otherwise making milk will drain her even more.
And, could you feed her maybe 3 times a day? And all soaked? And make hay available at all times, a good portion of it being alfalfa?
The mucus clearing sounds promising.
I do have a feeling her teeth are NOT okay, tho.....