Your other thread is linked below.
I don't see a tumor mentioned in the other thread.
Did the vet take an xray to see where the tumor was located and what it may look like?
I don't believe anyone can know if a tumor is benign or not without a biopsy.
Could be she has something going on with her gizzard, she may have a blockage in the intestines, tumor in the abdomen, internally laying, caseous exudes due to EYP or Salpingitis...there's so many things.
She didn't lose weight all in a day, so she shouldn't gain weight too rapidly. Give her the food recommended by your vet. If you wish to feed her something else, stick with poultry feed like chick starter or an all flock feed. Small treats of scrambled egg, fish (mackerel, sardines, tuna) or bits of meat can be given to boost protein.
It's good that she's acting spunky. I'd put her with her flock and let her chicken until she's unable to chicken anymore.
If you do lose her, please consider having a necropsy done through your
State Lab or perhaps by the vet, but you will want to get a written report of the findings, so you know what the cause of decline was and what the tumor was. Often going through your State Lab will be more economical and thorough than going through your vet depending on where you live.
https://www.backyardchickens.com/threads/sick-1-5-year-old-hen-help—what-to-do-next.1607622/page-2