Overheating after rear camera glass + battery was low before I disasse

So, today I got an iPhone 7. It has the rear camera glass broken. I didnt had the part so I thought I would remove from one of the iPhone 7 lying around for parts and would replace from there. I removed the whole rear camera cavity from the loner phone. Now I opened the customer's iPhone7 and disconnected the battery. Before even going fiurther I should mention that during the lcd removal I think I broke the screw on the right side plate where the earpiece flex snaps on to. It was loose when I opened it and I could see the black plastic thing loose also. I dont know how it happened. So, after this I disconnected the battery flex and the LCD flex also. No pressure on any of them. I put the LCD aside. Please note that the battery was only 13% when I started working on it. I cant remember now if I turned the phone off by sliding the power button on screen or not. Usually this is my habit but in this scenario I cant remember. Not sure if the phone turned off due to disconnecting the battery flex/cable.

Now, I removed the top metal cover from the camera and removed the flex and removed the camera too. Then I removed the black rubber underneath it and then the metal plate that holds the camera. I removed the broken glass cavity by pushing it out with a pen. Then I replaced this part with the part I removed from the loner phone by pushing it in from the outside and it snapped. Now I put back the rear camera (obviously with no metal plate as it breaks when you remove it). Now I put the LCD flex back and the speaker flex. Now when I am trying to put the plate back on the speaker flex (which is somewhat near to the camera flex) I see that one of the screw I can not tighten (as I mentioned earlier that I broke it). I tried my best to put it back as much as I can. Then I put the battery cable back. Since the battery was almost dead (around 13%) when I was working on the phone, I connected the charger. I pressed the power button to turn it on but it wont turn on.

Now, the phone get so hot near the rear camera and where the earpiece flex snaps on to and the IC (covered with the metal plate). I immediately removed the charger and removed the battery flex and removed the lcd flex and removed the rear camera flex. I dont know what happened. I let it cool down. So I tried to connect another battery and tried to turn on the phone and nothing happened. Then I connected the old battery again. Plugged the charger again assuming that the battery is dead and it needs charge. The phone do not get hot. Then I connect the lcd flex and phone do not get hot. I press the power button to turn on the phone but it do not turn on. Then I connect the camera flex and the phone gets hot again. I immediately removed all the cables again including the charging. I dont know whats happening. I have read so much on this site and saw only this thing happening and people saying its an iPhone 7 issue and do this and do that but no solution. My issue is that I did not put any after market parts. I put the same original LCD, same battery, same camera but why is this happening.

I also read somewhere here on this site that the broken screw could be a problem. Not sure if this is the reason.

Can someone please guide me or troubleshoot what is the problem and how to fix it. I somehow have given the customer a loner phone to work but I need to fix it asap or it will be my big loss as customer do not have insurance. Or atleast guide me what to tell the customer what the problem is and how to take it to apple store or anything like that.

I am xda-developers guy, gsmhosting guy but not apple guy :)

Thanks