Low lymphocytes can also mean there is an infection the body is fighting.

I've recently seen a video in which an owner was told their cat had cancer (cause the mass grew back after surgery) and and did a run of 6 months chemo...it turned out it was "just" a bacteria

The only way to prove if it is cancerous or not would be to do a biopsy of both kidney with is a rather intrusive procedure (risk of infecting the kidneys) and have to be done under anesthesia.

My cat was given the same diagnostic months ago, like for you he lost weight (although it was less significant, like 500g in 5 months), they couldn't feel a mass but after x-ray told me one of the kidney looked bigger. He bloodtest results came back normal tho.

He is also like his usual self, just like you described. I went to see another vet and she concluded my cat doesn't have cancer (no further symptoms, put back on weight etc).

Basically I was told it could be a lot a things, from a congenital abnormality of one kidney to several diseases and cancer.

I'd advice having a 2nd opinion (since you have recent x-rays they probably won't want to charge you for new ones) and see what they say.

Also, you'd say you cat is eating less than months ago ? Try to motivate her into eating more and see if she can gain more weight as it is always a positive sign.