The abdominal mass, combined with the probable heart issues, fairly much confirms she does not have much longer. I had a much younger hen (4.5 years) with an abdominal mass (smaller than that), took her to an avian vet, and after much consideration, decided to have her euthanised at that visit, rather than risking a potentially drawn out and painful ending. I got to hold her and stroke her as she drifted off. I do not regret that decision, but of course, was still extremely sad at saying goodbye.
I do not think the antibiotics will be helping in any way, not with what is going on with her (I could be wrong) . I would be more inclined to help her with analgesics, to help with any pain or discomfort she may be having. Frankly, if she were my girl, I would stop the antibiotics. They may even be making her feel worse.
Sadly, the only decision you have now, is to let her pass on her own, or if in pain or distress, have her euthanised. I found the decision to euthanise with some of my cats, they gave me "the look" of "it's time". Chickens are even more stoic, so you may not get a clear signal.
If she looks truly miserable, help her on her way (euthanise). Ask that you can hold her as she passes. The final goodbye is near, and the best you can do for her now is not let her suffer or be in pain.