Phone won't turn on and computer will not detect it, but gets hot?

I'm completely new to this community and I'm in a bit of a sticky situation and need quick answers before my deadline, so any help would be appreciated!

Here's my situation:

My supervisor at work was having issues with her iPhone 6 Plus and after a long conversation I told her that I could attempt to fix it for her. She said she had it worked on before at a repair shop in the next town over, but didn't really say what she had done to it. This time around, she told me she wanted her LCD screen and her battery replaced, and I told her that I was comfortable doing it because I had done it plenty of times before with my iPhone 5 so it wouldn't be a problem. Little did I know of the problems that would arise.

So I opened the phone up and immediately noticed that she had screws missing, one on the battery plate and another screw missing on the connector cover plates. Before I did anything, I tried disconnecting the battery with a plastic spudger but for the life of me I couldn't get it to come off--the previous person must have forced it back on. I eventually got it off with much struggle. Next comes getting the connector plate unscrewed. The first thing I noticed once I got the plate off and the screws placed carefully aside, was that the LCD flex cable was only halfway connected where the previous person once again forced it on and ended up damaging the tiny connectors on the logic board, so the cable itself couldn't connect properly. Despite this, her LCD display was still working. The reason she wanted her screen replaced was because when she tried to swipe down from the top of the screen, it wouldn't detect it, only part of the time. It would also type by itself. I figured that was the result of her LCD cable being only partly connected.

Just to be totally sure, I ordered her a new screen and had it put on, and once again I could only get the left side of the LCD connector to go in while the right side was raised. I reconnected the battery to see if the screen would work, and now the entire phone refuses to come on or even be detected by iTunes! I'm confused as to why it would work for her with the LCD only partly connected but when it's partly connected for me, it won't turn on at all. It gets hot while connected to the charger though, and the halfway plugged in LCD cable gets REALLY hot so I decided to unplug it.

Now, I've heard of a few other reasons why I could be having problems with the screen. Apparently, sometimes when you replace a screen it's normal for the LCD display to not work, but that wouldn't explain why even iTunes won't detect it. I've also heard that if the flex cable for the home button is damaged then screen issues could also arise. There's a possibility that I could've damaged that when switching everything over from the old screen to the new one, but I can't test that out to see because the phone won't turn on.

So I have several issues I'm trying to figure out while my supervisor patiently waits to get her phone back. I'd hate to hand her back her phone when it won't even work because when she handed it to me, it was working fine aside from her touch screen. It's like I've just made it worse.

Just something to remember, I'm am amateur at this and I'm trying my best to learn--experience is the best way.

Sorry for the long overload of information, I just wanted to clear everything up and be as detailed as possible for you guys to have a good idea as to what's going on.

Thanks in advance!!

EDIT: I've sent the phone off for mail-in repair because I wanted a new PCF connector for the LCD display and that required soldering which I'm not even going to attempt to do, but what do I do if the phone still won't turn on even when the connector is fixed?