Damaged logic board and USB connection doesn't work. What to do?

Hello,

A few months ago my phone fell at the beach, not directly in the water but in wet sand. Anyhow, it resulted in a little water damage near the charging dock and a broken screen. I successfully replaced the screen using the guide here. Fast forward a month and my friend accidentaly dropped the phone resulting in a second broken screen, which I again succesfully replaced.

Apart from that, since the sea incident, and more so since the second repair, I had noticed that the battery was draining too fast (lasting half a day), and the phone would turn off before it reached 0%.

At this point, I stopped using this phone and I gave it to a friend of mine. She said that the battery draining problem was getting worse, up until the point where it wouldn't turn on anymore.

Reading comments here, and having seen some corrosion near the bottom in my previous repairs, I decided to replace the charging dock. In addition, I also bought a couple of replacement batteries. I changed the charging dock using the guide here. That's the background, sorry for the long post but I wanted to give as much detail as possible.

Now the problem is that my phone is stuck in the DFU mode asking for iTunes. However, no PC, Mac, anything will recognize it. It seems to charge when connected to a wall plug. The power button and the screen, etc, also work, because I am able to turn it on and off and the apple logo and itunes screen appear just fine.

At this moment I am thinking of a damaged logic board. I have experience with electronics and work in a lab with all kinds of equipment. I downloaded the schematics from the internet and tested the logic board with a multimeter and most things seem fine. The VCC main net seems fine and has over 3V, the battery connector seems fine as well. I've checked some charging "fuses" and they seem to have continuity. All posible voltages (1.1 V, 1.2 V, 1.8 V, etc) are where they are supposed to be.

I think I've narrowed the problem to the usb signal lines. The ground and VCC lines are ok, VCC has 5V when connected to a wall plug or a PC. Even more, my windows PC recognizes that a USB has been plugged but it says that it malfunctioned and gives error code 10. I've also checked the two usb data lines D+ and D- and I think the problem is there. There are two test points for the usb dock in the logic board, TP9 is D+ and it gives me a reading of 2.8 V when connected to my PC which I think it's correct. However, TP10 (D-) has a voltage of only 50 mV. When connected to a wall plug TP9 has 3 V and TP10 has 200 mV. Can you point me to where in the board or what component may be damaged/broken?

I suspect there is a short somewhere, possibly due to some corrosion. At this point I haven't cleaned the board, but I plan on soaking it in alcohol if you suggest so.

Sorry for the extremely long post, any help would be greatly welcome. Thank you very much!