hi first of all welcome to you and Chewy to our forum, unfortunately not for good reasons...
this is a hard decision for all of us cat parents to make when faced with tumors or any other devastating disease.
I can't really tell you what to do that has to be what you are comfortable with and what you feel Chewy can tolerate.
I will add though to your worrying abt her hiding and running from you after the surgery, for the first days/weeks she will be on pain medication and will be quite sedated and therefore be calmer and sleep most of her days....so that will help. there are also some remedies that can be given to help calm her..I can suggest 2 that are very safe to use and are NOT medications...
they can be used even with the pain medications..
I will also give you a link to a journal written by a member last year for his kitty Lucy who underwent tumor removal on her mammary gland, she did very well and was still doing good on his last post in Dec./12...
here is the link, you will have to read it in reverse order...as the last post he made shows up first..
good luck, keep posting and we will try and help you with any questions or even just general chat anytime♥
http://www.medhelp.org/user_journals/index/2193335?personal_page_id=2691122