I would check the SMART status of the hard drive. If it has bad sectors, it could take a long time to boot into Windows, making you think there's a fault with the screen. An easy program to check the SMART status is Crystal Disk Info. If it's anything other than blue, I consider replacing the drive and installing the OS. If you have a linux boot disk, the disk utility in Ubuntu or Mint will tell you if you have any bad sectors and you'll see if it goes through POST and boots into the OS in a faster manner.

You can use contact cleaner on the circuit boards. I use it all the time when taking apart laptops. https://www.amazon.com/Max-Professional-2015-Contact-Cleaner/dp/B004PHDLQY/ref=sr_1_5?ie=UTF8...-5&keywords=contact+cleaner

I see the NUU X4 for sale brand new on ebay for $66. It's probably going to cost at least that much to fix the phone since you'll probably have to replace the digitizer to get to the headphone jack, not taking into account the fact you won't be able to get replacement parts for this device. A phone that sells for $66 doesn't have a market for selling repair parts. Usually for a phone that isn't iPhone or Samsung, your best bet is to buy a broken one that has a different broken part.