SteamOS is open source with some closed sources component. But most important think you seems not being able to understand is that Valve provide high support to Open source community, which means it wouldn't be surprising if they decided to drop a open source phone.
That's surprising, Thinkpad x240 owner here. With all the os i installed on it i never had to do such overcomplicated manipulation, just bios entering + boot on usb.
What did you edited ? Arch user here, never had this kind of issue. Also if you managed to install Arch, you should be able to fix it(maybe you switched from terminals, try ctrl+alt+1-9)
J'adore les bananes