[X Style (Pure) Q&A, Help & Troubleshooting] Oliie23:
I got that ****er to work. I got to the point you did and what I did was to physically disconnect the battery. The problem you're facing is that the phone is still turned on, which is why it's being seen by the PC as unknown device. The only way to get it working is to disconnect the battery to kill it, then to repeat the whole thing. Below is what I did to eventually get it to work, just in case anyone else has the same issues I did.

So first of all, I wanted to downgrade my phone from 7.0 to 6.0.1, since there isn't no 7.0 RETEU available for download. For the most part it works, but you can't flash the gpt and the bootloader from 6.0.1 (phone rejects it and gives preflash validation error). When I did it, it worked but I was stupid enough to OTA the phone to 7.0 which bricked it. What's important here is that the phone was running off a 7.0 bootloader, not a 6.0.1.

I tried following the guide in first post but I couldn't boot it at all. I eventually did and here's how:

1. I flashed the loader.img onto my SD card. I followed the guide in this. It's important to do it via the Oracle VirtualBox (I tried on a legit Win8 32bit but it didn't work for some reason, but Win8 32bit on VirtualBox did...). What's also extremely important and I had to figure it out myself is that if you're downgrading you have to use the 7.0 loader.img (posted by dzidexx somewhere in the thread). It's the only way to make the phone boot because that's the loader it used before. It's also important to use an external USB card reader, because it's the only one VirtualBox can see. It doesn't see internal laptop card readers. I got the absolute cheapest one I could find and it was fine.

2. The phone still didn't want to boot because the battery was depleted. I tried connecting to laptop but it didn't work at all. I connected it to the original charger and it actually showed the "bootloader unlocked" warning so it apparently wanted to boot but didn't have enough charge. It just blinked with that pic and immediately died. This showed me that phone has issues with charging and apparently the charger isn't giving it enough current (I think?). Then I used OnePlus dash charger (or Warp Charger as they call it now...). That one had enough power to make the phone blink a few more times. It looked like bootlooping because it showed the "bootloader unlocked" and then turned off, and then did it again a few times.

3. When it was blinking the bootloader unlocked I actually decided to hold just the VOLUME DOWN button and it actually entered fastboot which is quite the success! I think the original charger might work as well. It might need a little more time to get it to blink more, or perhaps try immediately getting to fastboot? I didn't, until I tried the OnePlus charger so I'm not saying the original charger won't work. I just came up with holding the button when connected the the OP charger.

4. From then on I left it at fastboot for a few minutes and tried rebooting. It didn't but I got it to the usual behaviour when your phone is completely out of charge and is charging - i.e. the white LED flashing with phone turned off.

5. After a few minutes the phone actually booted to the point where it'd show me the battery image instead of the LED which meant it's charging.

6. After a few more minutes the phone booted up. I left it to charge to 100% which took a lot longer than usual. But it was charging and that's extremely important.

7. Once charged, I followed the guide in original post step by step but since flashing gpt and bootloader gave me preflash validation errors, I didn't flash them. That resulted in the phone not wanting to boot without the SD card and showing up as unknown device.

8. I physically disconnected the battery and reconnected it again. I downloaded the 7.0 firmware from https://firmware.center/firmware/ but since there is no RETEU, I picked RETLA, but to my knowledge any of the versions can be picked, just so long as it's 7.0. Then I extracted only the gpt.bin and bootloader.bin and copied it into the folder with my 6.0.1 RETEU files (replacing the 6.0.1. bootloader and gpt of course). The point is that I needed a 7.0 gpt and bootloader and as far as I know, there are no region differences for these two files.

9. I followed the guide once again. Again, I required a freshly flashed SD with the 7.0 loader.img and that's how I got the phone to boot into fastboot. I followed the rest of the guide, this time flashing the 7.0 bootloader and gpt correctly (it worked without preflash validation errors). The rest of the firmware files were still 6.0.1. Like the guide says, first it has to be done on the SD card, then the card has to be disconnected and you have to do it again to flash it to the phone itself.

10. The phone booted correctly. It sees the SIM card and so far gives me no issues. It's running 6.0.1 while booting off of the 7.0 bootloader. A brave soul might attempt installing TWRP and through that getting one of the custom 7.0s but I'm not brave enough nor is there a fully working bugless custom ROM to my knowledge.

I hope anyone finds this helpful. Especially people who got to the point where I was. I thought the phone is gone for good but fortunately enough, I made it work again.

And for god's sake DO NOT OTA TO NOUGAT. I REPEAT DO NOT OTA TO NOUGAT 7.0.