Akash

Replace battery first and check that you can find the clue what you have to do.

Long screw damage must be check first.

Some where 3v connect to 1.8 that value shows 2.7.I also faced the same problem but i can`t solve it.I think under layer of the phone is shorted i think.

iPhone 6s and 6splus series have same issues that i solved on that cases that is the Chestnut related issues.First clear about that there is no light present but logo shows or not that is not the software issues anymore.Some capacitor near LCD connector causes that problem.

There is WI-Fi antena that might be open from the connector,i saw many times on many phones that the connector is open from board.So you need to open the phone and see first or try to use new one.If the problem is same after that is the cause of WI-FI chips.Follow this ifixit guide.

That is the problem of SWI line see resistance near battery connector might be open.This SWI line is connected to Tigris then CPU.

That is the easy way to find that shows error 9. 1- From Nand that can be solved by removing and reprogramming by programmer 2- Loose contact between CPU & RAM. you tell that phone working after heat up board is that the problem from CPU to SDRAM loose connection that is the mistake of apple design in 6 & 6plus series phones that the error 9 cause.Such problem i solved many by this way but you have to understand that is slightly hard. you need micro soldering repair skills and instruction on it. The I2C line have broken cause that problem .

iPhone have to find the solution by see the error code during firmware upgrade via iTunes.First replace battery and restore because low battery cause that firmware upgrade failure. See the history why that the fault is appear like water damage or forcely dropped that you don`t mention here.

Yes actually you need to insert the old one because that one is compatibie with one like finger print in iPhone only like one in one phone . Absoulatly yes you can.

That means some pin are dry sold or pin loose of LCD Connector that the cause. when you press working fine that pin are connected.No problem by using the tissue to press but not the ultimate solution.