Do this first, peel the black tape back on the bottom of the battery and check voltage of the battery. You'll need to be around at least 3 volts DC to get the iPad to do anything. You can also check the battery connector plug on the motherboard terminals to see if your getting any current with the charger like this. I had a similar issue and it was the charging port. I checked the battery and it was around 2.7 volts DC (too low), and the battery terminals were not getting and charge, so I knew it was in that circuit. Hope that helps.
The speakers have to sandwich the charging port, it took me a minute to figure out how this worked too. There is a positive and negative terminal on each side of the charging port (front and back of charging port). Four terminals total (one on front left, one on front right, one on back left, and one on back right). If you have it like this now, you may have shorted the speaker circuit. See photos: Photo 1 is charging port, Photo 2 is charging port "close up" terminal (one side), Photo 3 is speaker terminals, and Photo 4 is the speaker terminals sandwiched properly between the charging terminals. Hope that helps!
Yes, same as iPad Air
NXP USB IC is actually part number 610A3B, not 610A38