If you selected the reason "buyer changed his mind", then once you close the cancellation yourself, you shouldn't get a defect.
However, the key thing is whether the buyer made the request through eBay messages. If he/she did ask for the cancellation via eBay messages, you should be OK to go ahead and finalize the process since you're past the 7 day limit for the buyer to have done that himself/herself.
You can check your Dashboard a couple of days after finalizing the cancellation, and if you see a defect (assuming your buyer did use eBay messages to ask for the cancellation), phone Customer Service and see if you can get it removed.
Remember though that defects won't affect your seller status until you have 8 defective transactions from 8 different buyers. Still, it is best to keep the defects to a minimum, since most of us are on a 12-month evaluation.