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Hello. I'm sorry to hear about your cat.
The first thing I would do, is make sure there was a thyroid panel or T4 run. This is to rule out hyperthyroidism. That would be my main rule out with weight loss, hair loss and skin lesions in an older cat. If that was ruled out, then I would proceed to other diagnostics.
Here's a nice set of write ups on hyperthyroidism in cats: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=516
If it's not hyperthyroidism, then I would biopsy one of the skin lesions. It doesn't sound like leukemia to me. It sounds more like an auto-immune disease that is leading to the skin lesions. The biopsy would confirm this. Treatment is then with steroids- Prednisolone +/- other immuno-suppressant meds.
I hope this helps, let me know if you have any other questions.