Hi there, it seems like your LCD and digitiser assembly is broken and needs to be replaced. iFixit has a repair guide for this: OnePlus 3 Screen and Digitizer Assembly Replacement You could source a screen online for a reasonable price and try to install it yourself, alternatively, you can contact your local phone repair shops for a quote or further guidance. Good luck!
Hi Tomer, If the connection on your motherboard isn't mashed or damaged then you should have no problem plugging this cable in. You could be attempting to plug the cable into the wrong socket on the mainboard, a mistake I see trainee's making when doing iphone 6 repairs and not putting the cables in the right spot. The cables should not sit behind each other, and will all neatly line up to where they're supposed to plug in. Try tucking the speaker flex cable behind the screen cables, or visa versa. Good luck! ~S
U1602 chip is responsible for rear flash. Check chip under microscope to see if it has pooped out any solder balls or anything of the like - any obvious burns or marks etc. Would be almost impossible to tell if that chip is the problem without replacing it, you can order off ebay for about 7USD and try. Good luck! As for rear microphone, can't help you there.
Why did you replace the battery in the first place? Have you tried the reset using a different cable/charging port? (maybe port was damaged during battery replacement?) Is there any pry damage around the battery connector? (component knocked off somewhere?) Have you tried a different battery? And lastly, do you get an itunes error? If so which error is it?