Worry wart or not, these and many others are wise questions to have answered by an experienced, board-certified (American Board of Plastic Surgery) plastic surgeon who does more than a few breast enlargements per year. I have been in practice for over 20 years, do hundreds of breast surgeries per year, and I still have some patients who request or require re-operation. Fortunately, very few, but my perfection badge has not yet arrived in the mail! To answer your posted questions briefly:
1) The main reason for re-operation is size change, so choosing properly is important. There are lots of ways for doing this--use as many as possible, but also listen to your plastic surgeon. See my article "What is the Right Breast Implant Size for You?" for more detailed information. Other reasons for re-operation are bleeding, infection (requires removal of implants in most cases), malposition, and capsular contracture. My re-operation rate is less than 5% for all reasons.
2) Capsular contracture is caused mainly by bleeding, or bacteria, so surgical technique is critical. Good surgeons have capsular contracture rates less than 5%.Surgeons whose patients have significant post-operative bruising, swelling, and pain, or who routinely use a drain for breast augmentation cases, tend to have much higher capsular contracture rates.
3) Breast pain is mild after surgery and virtually always goes away completely with time. Contracture can cause pain in addition to firmness--that's why it is best to avoid this in the first place. See #2 above. Chronic breast pain (from any cause) is exceedingly rare!
4) The implant has to be in a space within your body, so some sensory nerves are unavoidably cut, regardless of surgeon, incision placement, or care during surgery. Most sensation recovers over time, but 5-10% of women may have permanent loss of nipple sensation; if a breast lift is also performed, the rate of nipple sensation loss is around 15%. When numbness occurs, this does not affect the ability to breast feed.
5) Saline implants can eventually leak and deflate, more common with textured implants, and more common in implants that are under-filled. The fluid is IV saline so this is not a problem, but the deflated implant must be replaced via surgery. Free implant, sometimes free surgery, but re-operation required.
6) The present generation of silicone gel implants (by both manufacturers) are cohesive and cannot deflate, leak, or rupture. MRI is incorrect 21% of the time (FDA data), and with these newest implants is unnecessary.
7) See #6 above. Older silicone implants had a more sticky silicone gel filling (less crosslinking and therefore not cohesive), and could rupture and cause inflammation or capsular contracture requiring surgery to remove the capsule and replace the implant(s). Use only the newest generation of silicone gel implants. See #6 above! BTW, you already have microscopic molecules of silicone in your body from needle lubricant on any shot you have received (like tetanus booster, immunizations, etc.), or intravenous lines. Less silicone will come off your breast implants than you already have in your system. For more information about this, read my article "Are Silicone Breast Implants Safe?"
I have written an 23-page informational article that I give to my breast implant patients. Many more questions are answered; but nothing beats asking a qualified, experienced plastic surgeon!