I'm also a runner, halfs and marathons and I'm doing a 100k over 2 days in July.
I've not had mx with ld flap but I have a small silicone implant on one side. Inserted 10 years ago after a wle 22 years ago.
I wasn't a runner per se 10 years ago but I was 'sporty' (squash, running for general fitness).
I am (was) a 34B so never invested in super serious sports bras. Sports bras certainly but not of the 'shock absorber' range. However since upping mileage in 2013 for marathons my implant started playing up and by the end of 2013 I asked to see a PS as it was feeling weird. I have capsule contractor. In 2014 it was feeling worse and I intended to get another PS appointment in early 2015.
But I got a mammogram recall on NYE and an now a 34A on the none implant side!!
To resolve the contractor and even me out again I've asked for it to be removed which has been agreed one this new, primary, bc treatment is over.
I found it more comfortable doubling up on bras. The best shapped support I found on the high street is M&S super high impact for running, a double pack, one black, one white. Then I wear a compression bra on top. I use a Nike super support one.
It certainly keeps my top half warm!
I can't answer points 1 & 2 regarding the surgery specifically but would expect everything to feel different, certainly in the short term.
Point 3. Rubbish. You can and will be back. Possibly even stronger as your mental strength to push on running will increase cause pain and tiredness while running is nothing compared to fighting cancer. BUT you need to give yourself plenty of time to ease back in and set sensible goals along the way.
(Says me, 11 days post second wle and about to go out for 10 miles... run/walk/run routine... to try and get enough miles in my legs for London on 26th April.. eeeek)