Only an ACMT would suggest trashing a $700 board over a simple circuit fix! First and foremost, ignore Adam Kruszka; he is Apple certified. This is a big red flag, as Apple certified technicians are clueless when it comes to most repairs. Their answer by default is to replace everything and you will see here this is no different. This is a retina; resale value over $1000, very much so worth fixing, and parts are still available to do a proper repair!
First thing's first, don't use rubbing alcohol. Put the board into an ultrasonic cleaner with 103 milliliters of branson EC gallon of distilled water. Ultrasonic for five minutes per side. Then, dip into a small tray of 99% alcohol, the alcohol with displace the water and evaporate quickly. After that, put it by a fan or an air filter or an air conditioner where it can dry. Then after that put it in an oven at 225f for a few hours. This is NOT to "reflow the solder", that is a BS internet myth that fixes nothing, this is to extract all moisture from the board.
If you can't get 99% alcohol, go for 91%, but nothing less!! No rubbing alcohol. If no ultrasonic cleaner is available, just light toothbrushing while in a baking tray of 91% or 99% alcohol will be the next best thing. You can get baking tray at any corner store deli or grocery store for under a dollar.
After this is done, wade through the internet to find a schematic and a boardview for an 820-3332 motherboard, if your machine is a 2012 15" retina. Also find a multimeter that allows diode mode measurements, and we wil start the journey of figuring out what's dead on your machine. I hope we can get this back to your original issue which was likely either a no backlight issue or just some corrosion on U9000, and you'll be back to having a working laptop.