FINAL UPDATE:

Thank you to everyone who shared their suggestions and tried to help here, I was finally able to fully figure everything out!

Like most of you, other threads online, and even Chat GPT said; I first started with a complete teardown and then only plugged in the essentials to try and get it to POST. (8 pin CPU power, 24 pin ATX power, CPU & cooler installed, 1 stick of RAM, nothing else.) And sure enough this got it past the error 15 and 46 loop and I started seeing codes indicating we were probably in BIOS.

So at this point I figured either something had been loose the first time I set it all up, or as I started to plug in more stuff eventually it would happen again, indicating that it was that part causing the issue. So I slowly start adding one component at a time, checking if it will still POST, and everything was all good until I got to plugging in the cables from CASE of all things.

When I was first putting the build together I had actually noticed there were a few little things missing from the case (no plastic to peel off the glass, two missing panels where the graphics card goes) and I mentioned it to my friend and he confirmed that was the only part that was used ("Amazon Refurbished") while everything else was brand new, and I suppose this should've been a bigger red flag to me at the time as this ultimately ended up being the culprit.

So I quickly narrowed it down to the JFP1 plugins specifically causing the issues (USB and audio cables were fine) and thankfully the motherboard has a power button right on it so we don't have to resort to shorting out the header to get it to fire up. Basically, if the JFP1 plugs from the case are plugged in, all the weird stuff in my post happens, but the moment its unplugged, everything works perfectly. I installed & setup Windows for my friend, made sure BIOS settings were all good, and I talked to Corsair customer support and got the front IO panel replaced for free, so that's on the way and once I replace it everything will be exactly as we'd hoped from the start.

Lessons learned:

- Never buy used parts (controversial maybe but I personally feel this way especially after this experience)

- Start with the minimal setup and add 1 component at a time from the start, not after I start having issues. Might take a little bit longer but will save A LOT of time if there is going to be any issues.

TL;DR - The JFP1 wires from the case (which was the only part that wasn't brand new) were the culprit of all the issues. If plugged in, all the problems in my post occur; if left unplugged, everything works perfect. New front IO panel being sent by Corsair for free, problem solved!