It seems she lost a fair bit of weight before you started the antibiotics according to those pictures? Infection is a very high metabolic state and results in lots of calories being burned. The cattle I used for my PhD research got some infections and they lost heaps of weight. I had learned about this in class but it was amazing to see it in action.
Antibiotics will kill the beneficial bacteria in your horses gut leaving it less able to digest its feed especially forage. This on top of an older horses inability to digest feed efficiently anyway puts the older horse at a real disadvantge.
So your horse had 3 things going against it 1) the high metabolic rate of infection, 2) it lost its good gut bacteris so couldn't digest feed as well 3) she's older and isn't efficient at digesting feed anyway.
As everyone has been saying, this will take time and you obviously are slowly winning the battle. You could consider a probiotic to get the good bacteria working again. I have seen good results with Forco and its relatively cheap. I have heard of some people using plain yoghurt but your horse might really turn her nose up at that if she doens't like sugar beat.
As you come into spring and the weather improves she should continue to improve. If you have access to grass in the spring that will really help.
On an aside when I was a kid we have a pony with terrible arthritis we gave him Rhus Tox. Its a homeopathic treatment for artheritis and it worked a treat. I don't know of the science behind this but know we personally had good luck with it. He went from being unable to be shod behind to being shoable for another 2 years. Eventually it didn't work and we switched to bute.