How to fix invalid serial number in "About the MAC" causing iCloud, apple ID issues?

I have a 2012 13" Macbook Air which since being upgraded to El Capitan is experiencing hanging issues with iCloud, FaceTime, family sharing, iTunes etc during sign in. It literally will no longer log into any accounts. I checked family sharing, user admin setting, checked notifications on, cleared plist's and tried to recover from old back up but after substantial investigation of forum discussions I noticed that the serial number in "About this Mac" is 17 digits and is also different from the etched 12 digit serial number on the hardware.

Thinking back 2.5 years ago the macbook had a motherboard failure while under warranty which was replaced in Apple store in Canada so I am thinking this is where the invalid serial number originates from. I have since moved from Canada to Brunei which has no Apple shop and now it is not under warranty or apple care support.

I do remember issues with FaceTime log in hanging which did not bother me as I typically use iPhone or iPad for FaceTime Up but I never experienced this level of problems. I see potential solution out there in forums for when the serial number is blank but they are clear they do not work for deleting invalid serial numbers and replacing it with the correct valid 12 digit one.

Anyone got a way to solve this?