Weird display behavior after reassembly and thermal paste application

Hi,

So I took apart everything and re-applied the thermal paste. Something's definitely wrong after the logic board reassembly; any help to figure it out will be appreciated!

History : I changed the SATA cables twice and figured they were getting destroyed due to overheating, so I'm currently running my SDD on an external USB casing.

This also means, that right now I have a loose, faulty SATA cable (and hence IR sensor/sleep sensor (?) ) inside the machine.

Symptoms :

First, the display didn't work at all. I followed the steps at

No video on lcd, but has video on external monitor

and figured it has something to do with the sleep sensor. When I took out the battery, SATA cable, and BIL and then reattached the battery, the display worked. Then, I tried attaching them back, and the display still works! Here, the laptop was in a vertical position, i.e. standing on one of its sides.

A minute later, I put on the screws, happy, and in the normal horizontal position, it stopped working again.

Right now, I booted it in a vertical position, and turned it around, and it's working, but something is definitely wrong, and I don't know if it will boot again normally in an hour.

Extra notes:

I checked the backlight fuse with a multimeter for continuity and it is OK.