I had this problem on a couple different S7 devices in my shop. Almost always after a device wipe. I tried a lot of aftermarket replacements and none of them worked. What DID work was replacing the rear camera with one from another S7 I got that was run over by a truck so they actually just donated it to my shop after I recovered data. I searched a lot around on troubleshooting resources and kept ending up with it being a firmware issue with the camera itself
I think it would be easier to use software that's available out there to put the phone into DFU. Unfortunately I don't have access to my computer that has the pin layout anymore or I'd just give em to you, but that's something to consider.
It's an old device with an old battery. Your battery could have just bit the big one after being used so long and the drop was just the straw that broke the camel's back. Replacing any component requires removing the digitizer, and if it's cracked that means a full replacement. If it's responding at all when you plug it in your charging port is likely fully connected, and it actually has a retaining bracket over it. In your situation I would suggest a battery and digitizer replacement.
That sounds like the connection to the display is bad. I have fixed this before just by cleaning the connectors before. Try this guide to get to the connectors: http://support.hp.com/us-en/document/c04165794
Can you confirm that windows is registering the drive? Also make sure that you don't have it assigned to a RAID. A customer brought me a computer he bought from a friend that was having this same issue, and the drive was assigned to RAID 1 rendering the drive inoperable, and in most cases looked like the drive wasn't even there, even though it showed up in BIOS.
It's possible that whoever installed the radio ran it through the power steering assist fuse, or some newer vehicles even have electric steering, causing fluctuations in the power being sent to your stereo.