I do not have a MacBook air, I do have an iMac running macOS High Sierra, I can only offer two suggestions.
My iMac has Time Machine as it's 'routine' backup facility. Does the MacBook have a similar facility and can you use it to revert the MacBook to time before you made these changes?
Secondly, the Apple support chat facility is truly exceptional. You can echat to them, you can ring them or they can ring you (and they will ring you 24/7 - no doubt about it). If no one here can help you I thoroughly advise using their support operation.