I had the same thing (firmware-lock and Icloud-lock) and I use a lot of linux distro’s, so I had the feeling of a procedure that would probably work. We’ll it did!
Important! Only if u don’t mind losing the files on your ssd or hd!
1. Download a fedora workstation live-iso https://getfedora.org/
2. Burn the .iso to a usb-stick. With Etcher https://etcher.io/ it will probably always burn successful, and available for win10, mac and linux.
3. Download a ubuntu live-iso https://www.ubuntu.com/
4. Burn it also to a usb-stick with Etcher.
5. Fedora uses also efi, like macOS. So if ubuntu will not show up as a, .Windows install-disc when pressing the alt-key at start-up. Then first boot in fedora live. If u can boot straight to ubuntu you can go to step 7, but it can’t harm to follow step 6 also.
6. Use the Disks utility in Fedora Live and delete all the partitions on the ssd/hd. Reboot.
7. Boot into ubuntu. And actual install it on the whole ssd/hd. Because we still need to replace the apple-efi-recovery with grub2-bootloader. In the install-proces you will get a warning like: “When proceeding you will delete efi. It can be difficult to install another OS later on the machine” We’ll just press ok, and accept the grub2-install of ubuntu. Reboot.
8. Hold the command-R keys at start-up. Make sure u can connect to wifi. Internet-recovery will start.
9. Open disk-utility and just format the ssd/hd back to 1 Apple journaled GUID-partition
10. Select fresh OSX install. Done! A fresh MacBook Air that u can register with your own Apple-ID.
I only tested it on Macbook Airs shipped original with Lion but I can’t think of a reason why it will not work on newer ones. Older ones (pre-internet-recovery) U probably can use a OSX install-usb at step 8.
Please add comments, when u run into some kind of trouble! Good luck!