I had one like this and i replaced Q5501 it got like thst because i did a GPU replacement. Check Q5503, C5551 and U5550 one of this 4 could be the reason of your issue. Well i had the issue in a 820-2915 MacBook Pro 15.4” 2010-2011. The sensor i replace was part of the GPU Proximity / GPU Die / Heat Pipe / Fin Stack circuit, so you can check something similar in your Schematic and do some testing. Good Luck!
for those that removing TRACKPAD FLEX from connector (J5800) and the issue is still there, remove CAMERA FLEX too, the SMBUS_SMC_A_S3_SDA goes to camera connector (j3402) pin 4 and SMBUS_SMC_A_S3_SCL pin 5.
In my experience can be 3 differents things: 1. The volumen flex is bad 2. The power button flex is bad 3. Water on the board The best way to test this is opening the phone and disconnecting the volumen flex from the board and check if still is not working properly. Do the same process with the second flex, if still the same problem probably water damage but almost always is one of this two flex. In all the phones or tables is the same process, the difference on same models is that sometimes the flex are solder on the board or is a contact flex but the idea is always the same.
Yes, you can do it, for the 32 bits and 64 bits, the thing is you need a compatible or programmed Nand, baseband and eeprom and they have to be from the same board in that way you have to solder again in the new PCB and in that way it will work. For the WIFI products like iPads and iPods only changing the Nand will work, and YES there's a special tool to reprograming the imei, and yes you can create a new completely imei and still would activate in the activation process.