I had a similar situation with my Pumpkin who was about 13 years old when she started having similar issues to your Isis. She just didn't seem like herself and was hiding a lot under the bed. I took her to the vet and he also found weight loss despite eating a lot, elevated liver enzymes, enlarged liver. The vet told me it was very likely some sort of cancer, and I took her in to the animal hospital where they confirmed the diagnosis of lymphoma.
This hospital had a very aggressive chemo treatment option, and since she was already pretty advanced in the lymphoma, this is what they recommended. They said Tapazole would not be very effective for her in where she was. It was weekly and lasted 3 months, alternating two different drugs, and there would've been a lot of additional medications she had to take, including appetite stimulators, anti-nausea, etc., all of which would have to be administered by me. I was terrified and apprehensive but decided to do it to give her a chance. She was already having some trouble breathing due to the fluid build up in her chest cavity, so she was kept in the oxygen cage for several days. Long story short, she seemed to recover slightly after the first treatment, and I brought her home after 5 days. Unfortunately that night, she went into respiratory distress again and we went back to the hospital. I decided not to go through with the chemo, because she was suffering and she really hated the car, the carrier, taking pills, and dealing with all that. Hardest decision I ever had to make, but I tried objectively to assess her quality of life having to get in the car weekly, getting drugs, taking a lot of pills, having trouble breathing. I was fortunate in that money was not a consideration for me, but she was further along than it sounds like Isis is. I think it makes sense to do a month on the Tapazole and see how she responds.
Finally, I found a wonderful Feline Lymphoma Yahoo Group that has many knowledgeable people who are extremely well-versed in the disease, treatment, and drug protocols that exist, as well as being a great support group. They were really helpful to me as this was my first pet and I felt overwhelmed, I believe there is also a vet that specializes in lymphoma who frequently answers questions as well. Here is the link, I hope this is helpful:
(Note: although it says they focus on young cats, I found the group to have equal representation from owners with older cats.)
You and Isis are in my thoughts.