I've had both these problems. A: The phone cannot be recognized by my computer (even though it was the day before) and B: once I pull the charger, the phone still thinks its charging.
My experiences:
A: I doubt this is a universal cause to the USB mounting issues and that this "solution" (if you'll let me call it that) will fix all instances of this problem, but I too scoured the forums and found nothing. I hope this helps.
I'm a scientist and have access to an exploratory microscope and fine tweezers. I took a look inside my USB port after cleaning it "blind" with alcohol/Q-tip/Tweezers and found it to be pretty clean. There was a tiny piece of copper (likely a pin) that had been mashed down and looked obviously out of place. I saw the pin had a twin on the other end of the port so I tried to straighten the pin with the tweezers and ended up breaking it off. Keep in mind, this was all done under a microscope. I was petrified that I had complete broken the port but when I tried connecting it to my PC, it was instantly recognized like the good old days.
Very strange. Maybe worth a try? Maybe I'm too late and you've replaced the phone? Let me know what comes of it.
B: After cleaning the port and powering my phone back up only to see that cleaning did not resolve the "am I charging or not" issue, I powered back down, pulled the battery, and bent the USB pin adapter away from the back of the phone towards the front. After powering back up, it worked fine. I've had to do this twice now with my phone so I assume this isn't a permanent fix but if you don't have insurance and are facing replacing the phone at full price, its worth a try.