1. Buy the Bosch part they sell to Porsche and take it to any muffler shop and for around a half hour labor they will fit it for you. More info and part number
here. ~$78 at Amazon.
2. How bad is the leak? Can you live with it? There is a new improved part (Around $40 IIRC) but you have to remove the transmission and, because it is a TIP, the labor hours are much more than they would be for a manual transmission.
3. The motor may be shifting because the front motor mount is worn through. Not an expensive part.