According to the Dell Support page that beep code is consistent with a Power Rail failure and the solution for that would be to replace the system board which isn't the most pleasant answer in the world, sorry about that. I did however see that a few users on Reddit had success resetting their RTC, now this very well may not work but it wouldn't hurt to try before replacing the Motherboard on the computer. How to reset RTC How to Reset the Real-Time Clock (RTC): (1) Turn off the computer. (2) Connect the AC adapter to the computer. (3) Press and hold the power button (around 30-35 seconds), until the power LED blinks three times. (4) Release the power button. (5) The RTC reset is complete after the power button is released.

No that is definitely not true, the black spots may even gradually spread, is sounds like the repair technician may have installed a bad display or damaged the display during repair, which can happen, but the repair shop should make it right and install a new display!