Question: This is a two part question: firstly if Crazy Carl was onto Sonic why didn't he find any of Sonic's quills when Tom found one without even looking for one? Secondly: what happened to the ring that Sonic threw and bounced of the robot, did he get it back, or will it play a part in getting Robotnick back in the possible sequel?
Question: How can Sonic's quill help Robotnik get back to Earth?
Answer: Robotnik is a genius inventor and the quill is an incredible source of energy, so presumably he will use it to power whatever invention he can create that will help him escape the Mushroom Planet and return to Earth.
Thank you.
Answer: First, it seems Sonic doesn't shed his quills that easily. Tom found one just by sheer luck. Second, yes that ring is likely going to play a part in the sequel.