Ouch! What you’ve proved is the backlight is not working so either your system is facing the dreaded FlexGate problem where the ribbon cable running between the T-CON board and display has failed or the backlight power logic within the main logic board has an issue.
[https://support.apple.com/13-inch-macbook-pro-display-backlight-service|13-inch MacBook Pro Display Backlight Service Program]
Your time is running out so I would get to an Apple Store or authorized reseller ASAP!
References:
* [https://www.ifixit.com/News/12903/flexgate|The Design Flaw Behind MacBook Pro’s “Stage Light” Effect]
* [https://www.ifixit.com/News/13979/apples-2018-macbook-pros-attempt-to-solve-flexgate-without-admitting-it-exists|2018 MacBook Pros Try to Solve Flexgate Without Admitting It Exists]
If your logic board backlight logic is the issue, Apple will tell you you need a new logic board, you don’t!
You will need to find an independent Mac repair shop with the needed micro-soldering skills to fix your logic board which will be a lot cheaper!