intro says they're "perfectly symmetrical", so I'd imagine they're mirror images of each other

Can you post a question about this? I try to avoid troubleshooting in comments as opposed to the forum made for it. Remember to include any symptoms (what happens when you plug it in, try to turn it on, etc.) as well as what fixes you've already tried. The better the info you give us, the better we'll be able to help. :D

You won't need to replace the mobo, but yes, it looks like you'll have to remove it. This guide is pretty confusing right now IMO

i thought this was just diagnostic why are we soldering

I updated the text part, but can't add new pictures since I don't have one of these toothbrushes

yep, the serial is tied to the motherboard

Aren't these the speaker cables? I haven't taken any 2DS's apart in a while, but I'm pretty sure these cables connect to the speakers

Works on every 3ds and 2ds model

Can anyone who's actually tried the text portion of this guide confirm whether it works? As I understand it, all the data on a 3ds is encrypted with a console-specific key (movable.sed) to prevent exactly this. If it actually worked like this, wouldn't it defeat copy protection almost entirely since any o3ds encrypted ROM could just be copied as-is to a n3ds and run normally?

You don't. The author just duct taped a step to the end of the laser replacement guide and called it a day