I'd suggest replacing the power/volume flex cable. They are very delicate and sometimes it's difficult to tell if they are broken just by looking. The only other thing I'd try is to adjust the two screws holding the power/lock button down. Sometimes if they are too tight it will restrict the use of the button. I will say though that this remedy is more successful for the volume up/down buttons than the lock button. If you chose to replace it yourself, I'd suggest buying a couple new cables just in case. They can rip easily while they are being installed if you're not careful. Also, about 1 out of 10 will be dead in my experience so it's always nice to have another one just in case. To fix it yourself, find a video on YouTube so you can see how it twists. The twists on installing that thing are a trip.
I repair a lot of iPads and I have never seen, nor can I logically explain, why the iPad wouldn't charge or mount even if the LCD is bad. I can't imagine the LCD can corrupt the logic board into not charging or mounting. Even in the case of bad LCDs, I could still pull it up on the computer and charge it. It sounds like you did your due diligence in what you tried so I can only think that trying a different LCD (one that you feel comfortable giving to the customer) would be possible to get it to work. Have you tried swapping out the LCD cable on that ugly LCD (which works) with the newer LCD? I still cant see that being the issue but it would be worth a try before you shell out for another LCD.
flogo2003 is correct. I would add that the two copper points touch two prongs that are attached to the sync port. The sync port is plugged into the motherboard. So, technically, the home button operates independently of the digitizer. It is just attached to the digitizer with adhesive for functionality and aesthetics.
There are little foam blocks that keep the display assembly snug to the frame circled in the pic below. 18 sounds like a lot though. Other than that, if they are little square metal pieces they are likely the tabs from the bezel on the digitizer. Most of them, unless you are SUPER careful, will break off when you pull the screen out. When you get a new bezel/digitizer (I hope there was something wrong with the old one, which is why you took it apart) it should have fresh metal tabs on it. Let me know if that helps.
If the ipad is on while you are replacing the LCD it will come up dark like that. Try turning the ipad off (hold the power button and slide the slider on the top) and then turn it on again. If that is the problem, it should turn on with full light.
Its really hard to explain via text so I understand your frustration here. My best piece of advice is to start from the lower end of the volume/power cable. If you start at the bottom and work your way up to the power button, the crazy acrobatic twists that the cable has to make around the rear-camera area kinda work themselves out. That might not help but, aside from watching someone do it in front of you or back-tracking while pulling the old cable out (i'm assuming thats not an option), this is the best advice I can give. Maybe search youtube for a video of someone doing it. edit: http://www.youtube.com/watch?v=0-EcAPXhKZk Around the 4:50 mark you can kinda see how the cable twists.
Agree with the other answers here. The ribbon cable is exposed between the volume-lock button and the power button and is very susceptible to being torn. Especially if you are not using caution when lifting the digitizer off originally, I've done it plenty of times. I even had the ribbon tear in the same spot just from touching exposed adhesive. As soon as I lifted the digitizer to reposition it, the adhesive grabbed the exposed ribbon and ripped it right off. I circled where to look below. Update: A few questions to see if I can help further: 1. Have you adjusted/replaced/done anything to that volume/power cable at all? 2. Does the volume-lock and/or volume up/down buttons still work? Another issue I have run into is if you tighten the screws down too tight on this cable (be it the volume section or the power section) the buttons wont work because they dont have wiggle room. You might want to get below the rear-camera and try loosening the two screws holding the power button just a tad. One turn out, one...