I have always lurked around Ifixit because of their wonderful guides, but I have finally decided to join and help contribute. I am very experienced in computer/mobile hardware and would love to help whoever I can!
Often times when replacing a component in an iPhone, the user may install the screws in the wrong place. Since they are of different length, it may cause a short somewhere inside of the device. Make sure no metal pieces are where they shouldn't be, and try again. You can also try to boot the device temporarily without any of the screws you unscrewed to see if that is the issue.
Try to press and hold Command-Option-P-R on bootup until you see or hear something new. Also, do you have another Mac that is new enough to download macOS Sierra from the App Store? If so, you can use target disk mode to install macOS onto the iMac via another Mac.
I would refrain from doing that at all because iPhones are extremely finicky with their parts and you could easily destroy something with the wrong part. Especially in terms of display, the iPhone 6 and 6s are very different because the iPhone 6s uses a 3D Touch display, so if anything, that might be the least interchangeable part.
The backlight it most likely dead. Set the brightness to the maximum, then when it goes black, point a flashlight at the display and see if you can detect a picture. If so, the backlight is almost completely out the window and can only deliver a very little amount of light. A quick Google search comes back with many useful results here.
If you have isopropyl alcohol, try rubbing down the trackpad and see if that cleans any uneven surfaces. If not, you can purchase a cheap vinyl skin to not only cover the trackpad, but to also give it a nice custom look. I would not recommend anything like sandpaper or any manual polishing that can give the surface an even more uneven feeling. Dbrand makes very high quality skins that you can check out here for your touchpad.
This is unfortunately a semi-common issue that I have dealt with in the past. You can follow the guide here to attempt to repair the Boot Configuration Data (BCD), or you can install the same version of Windows on the same hard drive but on a different partition to rebuild the BCD completely, which has worked for me in the past.
If the coating is something that peels off piece by piece, not only will taking the existing coating be a huge pain, there is almost no chance of getting a legitimate Alienware touchpad coating, as it is something very specific. Your best bet would be replacing the touchpad or finding your own kind of 3rd party coating that you can cut to the size of your touchpad. This is a guide that shows you how.
You most likely have a broken screen which is resulting in the phone not being able to boot up properly, or the GPU in the phone has died. If it is the latter, there is no fixing it besides getting a new phone or replacing the whole mainboard. If the screen is the problem, that can be fixed using this guide here, but it won't be easy.
This is a very common issue when the screen has been user replaced. If the wrong screws have been installed in the wrong place, the screen might have a blueish tint with other issues arising as well. This is because the longer screws would hit a PCB on the other side. Try removing all of the screws you had to take out and try to see if that fixes the problem.