Hello and thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 25 years experience and would be happy work with you.
It sounds as if it is very possible that there is some degree of hepatic lipidosis and it is true that cats with HL often have an underlying cause that and it is not uncommon that the cause is not ever identified, but in her case, that seems very unlikely. The lethargy and jaundice cat certainly be explained by HL, but cats with HL alone do not have the other clinical findings such as enlarged nodes and pleural effusion. The enlarged liver is likely due to the lymphosarcoma and less likely due to HL, especially if she is eating. It usually takes days of anorexia for HL to develop.
If you wanted to have additional confirmation, a fine needle aspirate of the liver could be done. This is not as invasive as a biopsy and usually can be performed with a very light sedation (or in some cases, if the cat is lethargic, no sedation). Hepatic lipidosis is easy to diagnose with a needle aspirate and some cases of lymphosarcoma will also be confirmed with a needle aspirate. This may at least give you the assurance that you need to make your decision. Even if no abnormal lymphocytes were seen, if there was no evidence of HL on the needle aspirate, that would certainly keep cancer high on the list.
I hope that makes sense and is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.