I'm not an expert, but it sounds like a hardware problem. I've experienced something similar when a ram stick failed. If you have multiple ram stucks, can you remove one and try booting? This helped me diagnose in the past. UEFI I don't think goes bad too often, but ram fails a lot and is easy to check. Sorry that I can't help with your coreboot question.
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I asked the same question on Reddit, and peoples there seems to agree with you. Even if I could make my PC start again, an UEFI corruption isn't something that would happen without a component on the motherboard dying first. (In fact, the firmware might not be corrupted at all.)
A parameter corruption is a bit more susceptible to happen, (not by much, mind you) but would correct itself if I remove the CMOS battery long enough.
As for the RAM, I tried with another pair I had lying around, but it might not be compatible with this computer. (It is supposed to support RAM sticks of 8GB max, but I know some computers don't like when there is more than 4chips on each stick, and those 8GB ones have 8chips by stick.) I'll try each of its original 4GB sticks on each of its RAM emplacement this W.-E. I will post the result of all those last tests here, naturally.
Thanks for taking the time to read and give your opinion on my problem, it's greatly appreciated.