Did you remove the EMI shields when you cleaned the logic board? When you do iPhone water damage repair, the only true way to be sure that you are cleaning it is to remove these shields, otherwise nothing is really getting cleaned. Use this guide and this video by Jessa in order to thoroughly clean the device. Once you have done that, we will be able to troubleshoot components.
You will have to go to Apple with proof that you are the original owner of the device in order to get it unlocked. Otherwise, you will have to contact the original owner and get them to remove the phone from their iCloud account with these steps: 1. Go to iCloud.com on a computer 2, Click All Devices, select the device, then click Remove from Account.
To reset your passcode, you can restore your iPad Mini through iTunes. First, you will need to put the iPad into recovery mode using these steps. (https://support.apple.com/en-us/HT204306) 1. Disconnect all cables from your device. 2. Hold down the Sleep/Wake button, then "slide to power off" to turn off your device. 3. Press and hold the Home button and plug the device into your computer. If your device doesn't turn on automatically, turn it on. Don't release the Home button. 4. Continue holding the Home button until you see the Connect to iTunes screen. Then, connect your iPad to your computer and open up iTunes, then click Restore iPad and iTunes will restore your iPad (from a backup if you have one, otherwise to factory settings). Best of luck!
Is it the LCD or the entire display assembly? If it is the entire assembly, you can find the replacement part here and use the video guide here. If the LCD screen itself is the only damaged part, you can get that part here and use this walkthrough video.
I noticed you mentioned the 15" model but tagged the 13 inch. Assuming that you have the 15 inch model, here is the disassembly guide, and you can stop when you get to step 38. Here is the trackpad replacement guide (meant for the 13 inch, but it still applies). You can order the part here. Before you purchase the part, check to make sure the trackpad ZIF cable is properly seated on the logic board and is not loose, since that could also be causing your problem. These cables are sensitive and can become dislodged.