I have actually responded to both of your questions in other posts, nothing has been avoided, if you look back you'll see. But for discussion's sake:
Why have coaches covered their mouths?
players and coaches trying to read coaches lips have been around forever, but using the naked eye is different then recording. So the reason they have is out of fear that a player or coach might be able to read their lips and decipher what they're saying. It has not been from fear of videotaping because when the Pats were caught, they were the one and only team to ever be caught doing so, before then it wasn't a thought like it is now.
Why did the Patriots do so well after being caught?
This was because players such as Brady had aquired a great deal of knowledge from 6 years of signal stealing, thus becoming a more knowledgable QB and better at playing. Same for the coaches and other players, the knowledge aquired thru illegal means seasoned minds and a memorization and knowledge of the game amplified thru the material they intercepted. Also, the 2007 Patriots aquired more quality FA pick ups then I can ever think of, so those players played a vital role in bringing already established talent to a team of knowledgeable players. Randy Moss was the key, he is the best WR in football hands down. He's made a career of making QB's look better then they are. So those factors contributed to the Patriots going 18-1.
Hope this clears things up mates.