Your many questions indicate that you need to do a LOT of research before committing to this procedure. Google some medical studies. Although it works very well for some, many users report unsatisfactory experiences. Remember, what the surgeon calls "satisfactory" and what the patient calls "satisfactory" may not be the same thing. Some users report that the device helps some, but not enough.

Below is an extract from my draft book on managing incontinence.
The InterStim electrical stimulator device, formally known as The Medtronic InterStim Therapy for Urinary Control, was approved for use in the U.S. in 1997. A small device is surgically implanted under the skin in the lower back or abdomen. It transmits mild painless electrical pulses to the sacral nerve located just above the tailbone. The InterStim device reduces urge incontinence (the inability to defer urination or defecation). A handheld device is used to adjust the stimulation. The use of the InterStim is reversible and can be discontinued with no permanent damage although minor surgery is necessary to remove it. Minor surgery is required about every five+ years to replace the implanted battery. There are newer versions of the battery that are rechargeable and last much longer. The device reduced symptoms of urge incontinence in a majority of users, with about a third reporting significant reduction in incontinence episodes. About one half of the participants in one clinical study reported adverse events, including pain, infection, or movement of the wire, with one-third having the device surgically removed. Most health insurance will cover the cost of this procedure.
--John
(double incontinent)