Its possible that you caused a non-fatal short when plugging in the new screen. I have seen a short prevent the display from functioning even when replugged, and the device power cycled. The 2 ways I've seen devices restored to full functionality was a soft reset or a full cycling of the battery itself (drained to zero then charged until boot). Of course this only applies if you (one of the following) left the phone on/the battery plugged in/were not properly grounded when performing the repair.
Is the button (the part you touch) separating from the devices' frame? Or is the button (the interior mechanism that's activated by applying pressure on the part you touch) failing to work ie it clicks but does not always turn on/off when it does? What you've described sounds more like the former, which is good, because the fix is way easier: In most cases the button just needs to be re-seated in the frame/button assembly. You can do this by opening the tablet up and maneuvering the exterior button back into place from the inside with a pair of tweezers.
Sounds like either faulty connections, or failing hardware. Try unplugging your DVD drives' power and data cables and plugging them back in. Be sure to look for dust or other substances that might disrupt the connection. Btw did you use google translate?
Yes! Each model varies slightly, though it's enough to prevent a off model MB from fitting in your frame/making all necessary connections. Update (08/16/2017): Models G930A G930V G930T G930F G930FD G930P are physically all the same. The difference between them is just how they are programmed before shipment. I have an uncommon s7 model, so I always get model # specific components...
I would try disconnecting all of the antennas and cleaning anywhere they need to make conductive contact on both the male and female sides of each connector using a q-tip and 99% isopropyl. I have experienced finger grease interrupting both wifi and cell signal just from touching a connector while plugging it in.
It looks like the Galaxy Grand Neo Plus' charger port is soldered onto the motherboard, (see imgur link below for visual) this means your going to have to remove the port component and solder on a fresh one. I couldn't find any available with a quick ebay search, but if you open up the phone and count the pins connecting the port to the MB and get measurements for all sides you should be able to find an equivalent component. http://imgur.com/a/rk2OF
There is a possibility the LCD you received was defective, its is however much more likely that the (I think 2 off the top of my head) cables between the LCD and the Motherboard got pinched or yanked in a way that weakened them enough that the fall the laptop experienced 'finished the job' so to speak. I would recommend ordering a set of LCD/Backlight cables (they'll probably be pulled out of another laptop, they are exceedingly difficult to find new) and testing the new MB with the new cables and the old LCD. P.S. This info comes from going through the same extremely frustrating experience you're experiencing, only I ordered 2x new LCDs and 2x new cable sets before I got it over with