Hi Sheri,
I would not even think think about submitting a 17 year old for surgery.It may be a benign cyst, due to a bad tooth, or lipoma. Even if it is Cancer (God forbid) it is best to leave it alone unless she has difficulty eating.
A needle biopsy is not exceptionally invasive if you really want to know what this is.
I can tell you that if she were mine I would just let well enough alone.She sounds like she is happy, well and doing great.
Since it is slow growing I don't think three weks will change her prognosis.
I hope your baby will be fine.
Best wishes for her,
Terri
Feline Healthcare Expert
Expert in feline health and behavior. 20 years experience with cats.