Can't Get back UEFI and Secure Boot on XPS 13 9350 (Developer Edition)

Okay guys, this is really embarrassing and I already know I'm an absolute idiot so please, just bear with me.

I just bought the XPS 13 Developer Edition, simply because I already have an extra Windows 8 license key and didn't want to pay extra for Windows 10. Now I know that Windows 10 wouldn't accept my Windows 8 key but I also read that Windows 8.1 doesn't accept Windows 8 keys either. I really didn't want to have to install Windows 8, followed by 8.1 and then Windows 10 so I did a search and apparently, if I used a KMS Client Setup Key during the 8.1 install, I will be able to change the key back to my Windows 8 key after the install. So I gave that a try but unfortunately, after the install, it won't accept my Windows 8 key. Finally, I decided to just screw it and install Windows 8 and start from scratch. I probably should have read up on how to downgrade from 8.1 to 8 but I thought I'd get an option to format the disk and start from the complete beginning. Turns out, it just went ahead and started installing Windows 8 (on top of 8.1 I presume). After that, it wouldn't allow me to boot anymore, each time saying that Windows detected an error or something.

This was when I decided to boot into the bios and get it to boot from my Windows 8 USB drive to see if I can start over from there. For some reason, it wouldn't detect my USB and after some searching around, I realised that it's because UEFI doesn't support USB bootup (unless I'm mistaken) and that the only way to do it would be to go into the BIOS and disable Secure Boot, Enable Legacy Option ROMs and then choose the USB in the boot sequence. After doing all that, I managed to boot with my Windows 8.1 USB and install Windows 8.1 (after editing a file that allows the product key phase to be bypassed during the install) and activate it with my Windows 8 key after installation.

Problem is, now when I try to re-enable UEFI and Secure Boot, the computer won't boot and says that it can't find a bootable device. Can someone please give me a hand and let me know how I can get out of this mess I made? Thank you!