New 3DS XL won't turn on and will briefly blink red while charging

I bought a working n3DSXL (DS1) that had a broken top screen. When I initially received the unit, it powered on just fine, audio played, and the setup screen was displayed on the bottom screen.

I did a shell swap as well as replaced the screen and put everything back together. When trying to power it on, I first got the famous popping sound - blue right would briefly turn on and then shut off. I did some research and had read that this can be due to damaged or poorly seated ribbon cables, so I re-seated all ribbon cables and put it back together. I didn't see any visible damage to the ribbon cables.

Since then, the console won't power on at all. If I plug it into the mains, I get the titular problem which is that the console will charge but will briefly blink a red light if trying to power on. I've also tried a charged and known-good battery from another n3DSXL (DS2), but the same issue persists.

I tested for continuity on the ABXY board fuse - no issues there. I checked all resistors near the charging port + antenna hookup, so no issues there. I had read that sometimes a replacement ABXY board can help solve this issue.

I pulled the ABXY board from my known-working n3DSXL and swapped it into my broken 3DS. The system powered on with both the power and wifi light on, but both screens stayed dark and the volume was silent.

I then put my ABXY board back into my known-working n3DS XL, and now this one has the popping issue as well and won't turn on! :-(

So my question is this - why is it that DS1 won't power on, even with mains charging and a good battery, and that the components are the same and nothing is visibly damaged?

And why is it that DS2 now won't and has the popping issue just because I swapped its ABXY board into DS1?