Do you have a real name we can call you by? If not, Vegas is fine... Sorry you got the dreaded news, and whatever you feel is right...with all you have going on, don't fret about whether your feelings are valid or vane or anything else. Yes, I remember those days too, feeling like I was in a bad nightmare and could not wake up, very surreal and very frightening. Well November 30 marks my 7 year anniversary of that day that changed my life and I am thrilled to say I am still cancer free! I was diagnosed right before my 31st b-day with no hx of cancer either, it was quite a shock to us all.
As for your decisions, take your time. Don't feel you have to decide in just a few days, unless they tell you that it is so aggressive you have to act fast. MOst will tell you to take your time and research and go for a second opinion and think about what is right for you. No answer is right for everyone. I think a 2nd opinion would be great, perhaps they will offer you a more definitive answer.
I was offered a choice of mast vs lumpectomy, and had 2 surgical opinions plus 2 oncs, and 2 rad oncs, all supported the choice for me and that I had equal survival rates. I chose a lumpectomy. I was told that once you make a decision, don't look back....I did consider a mast about a year or 2 after treatment, but decided against it. Don't forget, you can have a mast later but it is irreversable.
As for chemo and radiation..it will depend on the specifics of your tumor pathology. Mine was 2.5 cm and grade III, chemo was not an option, but mandatory. Radiation was also mandatory for me since I had a lumpectomy and negative nodes. All that will depend on your node status, size and traits of the tumor and your surgical choice. Some people with a mast get rads too.
I would explore all the options, get more info and keep asking others who've walked the walk. This is a whirlwind of a time right now, there will be lots of new information and lots of doctors. COnsider taking a tape recorder to your appointments so you can ask what you need to and not have to worry about taking notes.
And stick around, we're here for you!