Hi Chris,
You certainly are having a rollercoaster ride. That's a good result to get an appointment so quickly. Good on you for being so proactive. It still gives you time to make a list of questions to take with you. Are you able to take someone with you to write out the answers?
On the top of your list should be the question of a sentinel node biopsy. As you previously pointed out, if it hasn't been done before mastectomy, then not possilble after. I'm not a medical expert - but seems to be alot of information out in cyberspace that recommends this action.
Don't stress too much about the gap between surgeries - my first WLE was 9th Dec 13, 2nd on 18th Dec 13 and mastectomy/reconstruction not until 10th Feb 14 - two months between first and last surgery. After each clinic appointment, the ops were scheduled between only a couple of days later to 3 weeks later. It's good to have time to prepare for the surgery both materially (all those bits & pieces you need while you're stuck in a hospital bed for a least a week if not more) and mentally.
take care
Linda