Another possibility is that the flex connector is getting dislodged during re-assembly. When you pull up the digitizer check for damage to the flex cable. If it’s ok, then re-seat the cable and test with the unit open to see if you get any response. I haven’t done gen 4, but on gen 5 the flex cable had to be folded just right for it to fit in without damaging it.

I suggest replacing it. The part is fairly cheap, less than $8 on eBay. The hard part is replacing it, but that’s probably easier than trying to repair it.

I went through at least 4 digitizers to get one that worked properly on an iPad 5. The bad units partially worked — they had dead spots on the touchscreen. Never saw one be completely dead, but I suggest trying again. Make sure you’re getting the correct generation/model digitizer.

I just re-powered an Ergorapido with a 2200mAh LIPO battery, like ones used in drones and RC cars, and it works great. My unit had NiMH (metal hydride) batteries in it and the original charger circuit for those batteries was definitely lacking. The newer models of the Ergorapido use Li-Ion batteries and should be better. Don't get the one with NiMH batteries. My original problem with the vac was anemic performance. For what it's worth I believe the original problem on my unit was caused by a defective power pack. If you see the blue "charge" lights blinking on and off a lot then you may have this problem. Get a correct replacement power pack from Electrolux to avoid battery damage or worse. Li-Ion batteries are very sensitive to charge/discharge and can be dangerous if not done properly. To use the LIPO battery pack I had to add a "Protection board" to the vac that manages charge and discharge rates and levels. It was a lot of work to shoehorn everything in the vac body and there were fairly extensive...

I've seen services on the web offering water damage repair for around $50. Worth it if you don't have the tools or skills needed to do it. Many times the phone can be repaired by someone who knows what to do. You can buy motherboards online too, but they're around $300. Watch out for the $10 boards -- they're BARE boards, no parts on them and are useless.

There's clearly a shunt somewhere drawing current. I would first check your reassembly to make sure all the parts got back together correctly. As I recall there's a piece of insulating tape near the camera - make sure that's in place. Another item to check is the connector to the display. Make sure it's clean and properly seated. The backlight power feed goes through that connector and it's one of the bigger power draws in the phone. Pull that connector and check with a magnifying glass for any possible shorts. Don't scrub the surface of the board. There are some very small SMD parts on the board. You can scrub some of those right off the board if you're not careful!