The mechanical valve will not wear out. But as your princess grows, her heart grows too, but the valve does not. Therefore she will probably need a replacement after a couple of years.
Once she has stopped growing, the mechanical valve will last her whole life and need not be replaced anymore.
Yes, lifelong medication :-( , but fortunately that normally goes without much problems.
You little girl will recover very quickly, especially with all the love and care she gets.
For a baby she may feel better much cooner than an adult. The valve needs to be replaced every 10 years. There are no disadvantages. It will help your baby breath better and have a better life! I had the surgery 2 years ago and I am glad I did.