Possibly someone inserting a USB device the wrong way and using force has damaged the USB socket. Or possibly leaning on the connection and bending the internal connectors. Or trying to insert an incompatible plugin device (can't think of any at the moment). Those are physical connection damages. I've seen damaged USB sockets and they are not pretty.
If it is internal then the USB chip has gone. Could be a power spike or a bad motherboard (faulty capacitors) or a faulty power supply.
As for point 3.) that would be a waste of time.
You'll get some more comments from other experts on this.
